Assuming you are referring to the area of a "trapezoid"; in which one calculates the Area, "A", as follows:
 A = 1/2* h(b1+b2) ;
in which: A = Area = 16 (given);Â
        h = height = 4 (given);
        b1 = length of one of the two bases = 3 (given);
        b2 = length of the other of the two bases = ? (what we want to solve                                               for) ;
Using the formula:Â A = 1/2 h(b1+b2)Â ;
Let us plug in our known values:
 →  16 = (1/2) * 4*(3 + b2) ;  → Solve for "b2".
 →Note: On the "right-hand side" on this equation: "(1/2)*(4) = 2 ."Â
 So, we can rewrite the equation as:
 → 16 =  2*(3 + b2) ;  → Solve for "b2".
We can divide EACH side of the equation by "2"; to cancel the "2" on the "right-hand side" of the equation:
 → 16 / 2 =  [2*(3 + b2)] / 2  ;  → to get:
8 = (3 + b2) ;
 → Rewrite as: 8 = 3 + b2;
Subtract "3" from EACH side of the equation; to isolate "b2" on one side of the equation; and to solve for "b2" :
 → 8 - 3 = 3 + b2 - 3 ;  → to get:
b2 = 5; Â From the 2 (TWO) answer choices given, this value,
"b2 = 5", corresponds with the following answer choice:
b2= [16-6]/2= 5 ; as this is the only answer choice that has: "b2 = 5".
As far getting "b2 = 5" Â from: "b2= [16-6]/2= 5"; (as mentioned in the answer choice), we need simply to approach the problem in a slightly different manner. Â Let us do so, as follows:
Start from:Â A = 1/2 h(b1+b2); and substitute our known (given) values):
→ 16 = (1/2) *4 (3 + b2) ; → Solve for "b2".
Note that: (½)*4 = 2;  so we can substitute "2" for: "(1/2) *4" ;Â
and rewrite the equation as follows:
→ 16 = 2 (3 + b2) ;
Note: The distributive property of multiplication:
a*(b+c) = ab + ac ;
As such: 2*(3 + b2) = (2*3 + 2*b2) = (6 + 2b2).Â
So we can substitute: "(6 +Â 2b2)" in lieu of "[2*(3 + b2)]"; and can rewrite the equation:
→ 16 = 6 + 2(b2) ; Now, we can subtract "6" from EACH side of the equation; to attempt to isolate "b2" on one side of the equation:
 → 16 - 6 =  6 + 2(b2) - 6 ;
   → Since "6-6 = 0"; the "6 - 6" on the "right-hand side" of the equation cancel.
→ We now have: 16 - 6 = 2*b2 ;Â
Now divide EACH SIDE of the equation by "2"; to isolate "b2" on one side of the equation; and to solve for "b2":
  → (16 - 6) / 2 = (2*b2) / 2 ;Â
   → (16 - 6) / 2 = b2 ;
    → (10) / 2 = b2 = 5.
NOTE: The other answer choice given:Â
"16= 1/2* 4(3+b2)= 6+2b2" is incorrect; since it does not solve for "b2".