Question

The rectangle below has a total perimeter of 125 cm:

Answer 1

A rectangle has four sides, two equal to the width and two equal to the length. So, the perimeter is

$p = 2w+2l$

If we know that the width is 5, and the perimeter is 125, we have

$125 = 2\cdot 5 + 2l \iff 125 = 10+2l$

In your case, the unkown is called x, but the concept is the same:

$2x+10 = 125$

where x is the length of the rectangle.