Okay, so you know what a rectangle is, right? It's a 4-sided figure with all 90° angles. The pairs of sides opposite each other have the same measurements. The perimeter is just adding the four sides together.

So if your rectangle is length x meters, this means 2 of the sides are x meters, and if it has width 2x-5 meters, this means 2 of the sides are 2x-5 meters. So we add x + x + (2x-5) + (2x-5) and set it equal to 50 to solve for x, because that's the length we're asked to find. If it wanted us to find the width, we'd solve for x and then plug that into 2x-5 to find that value.

x + x + (2x-5) + (2x-5) = 50

can also be written as

2(x) + 2(2x-5) = 50

2x + 4x - 10 = 50

6x - 10 + 10 = 50 + 10

6x = 60

x = 10

The length is 10 meters.

For extra credit, 2(10)-5 = 20 - 5 = 15. The width is 15 meters. And if we add 10+10+15+15, we get 20+30=50, thus confirming that our original calculations were correct.