A rectangle has four sides, two equal to the width and two equal to the length. So, the perimeter is
[tex] p = 2w+2l [/tex]
If we know that the width is 5, and the perimeter is 125, we have
[tex] 125 = 2\cdot 5 + 2l \iff 125 = 10+2l[/tex]
In your case, the unkown is called x, but the concept is the same:
[tex] 2x+10 = 125 [/tex]
where x is the length of the rectangle.