I'm assuming EFG is a triangle. In order to solve this problem, you do not have all the information to solve for x in a numerical form. However, if you let variable P equal the perimeter. You can solve for x in terms of P. Perimeter is the sum of all the sides of a shape.
First, let's add together all the sides of the triangle. (P = 5x+15+53+143).
Add together the common variables so that (5x+211=P).
Our next job is to isolate the x. We start by subtracting 211 from both sides. (5x=P-211)
Next we divide by 5. [x=(P-211)/5]
Therefore, [x=(P-211)/5], assuming P = perimeter
