Find the area of a triangle whose two sides are 10 inches and 18 inches long, and has a perimeter of 42 inches.

If the perimeter is 42 inches, then the length of the third side is

42 inches - (10 inches + 18 inches), or 42 inches less 28 inches, or 14 inches.

There's a "Heron's Formula" which is used in finding the area of a triangle when the lengths of the 3 sides are known (but the angles are not known).  Please look up "Heron's Formula."

You could also find the measure of any one of the angles by using the Law of Cosines.  Once you have the measure of one angle, you can find the remaining angles using the Law of Sines.  Please look up these Laws.

It's up to you to decide which method to apply here.
                                                                                  a+b+c
If you choose Heron's Formula, then calculate S = , where
                                                                                       2
a=10, b=14 and c=18 (inches)

and then evaluate the following:

Area of triangle = sqrt( S(S-a)(S-b)(S-c) )

Hint:  S = 21 inches in this problem.