A rectangle of length L and width W and a square of side x have equal perimeters. The length L is 10 more than x. Express W in terms of x.

We'll write the perimeter of the rectangle:

P1 =
2(L+W)

We'll write the perimeter of the
square:

P2  = 4x

We know that
P1 = P2 => 2(L+W) = 4x => (L+W) = 2x

We also
know that L = 10+x

10+x+W =
2x

We'll isolate W to the left
side:

W = 2x - x - 10

W = x -
10

The width W expressed in terms of x is: W
= x - 10.