The dimensions of a rectangle can be expressed as x-3 and x+8. if the area of the rectangle is 42 square feet. find the dimensions of the rectangle.

We know that
the area of a rectangle=length*width
let
length=x-3
width=x+8 
area=42 ft²
so
42=(x-3)*(x+8)----------> x²+8x-3x-24=42--------> x²+5x-66=0

using  a graph tool-------> to resolve the second order equation
see the attached figure 
the solution is 
x=6
hence

length=x-3------> 6-3-----> 3 ft
width=x+8 ------> 6+8----> 14 ft

the answer is
the dimensions of the rectangle are 3 ft x 14 ft