The dimensions of a rectangle are its length (L) and width (W), so the sum of its dimensions is (L + W)
The perimeter is the distance around the outside of the rectangle, or L + W + L + W which we can simplify to 2L + 2W
Now we can set up an equation to solve the problem.
The perimeter (2L + 2W) is (equals) 25 less than three times the sum of its dimensions.
2L + 2W = 3 x (L + W) - 25
2L + 2W = 3L + 3W - 25
-L - W = -25
Multiply both sides by -2 to get
2L + 2W = 50
Why did I multiply both sides by -2? Because the problem asked for the perimeter which is 2L + 2W
So the perimeter is 50. Note that I was able to solve this without ever knowing the length or the width!
Hope this helps.
