Explanation:
the perimeter of a rectangle is
2×length + 2×width
simply because the perimeter means to go along every side the shape has.
I don't see any answer options. you did not provide any. I can only give you the approach of how to solve this.
so, that means for us
2(6x + 5) + 2×3x = 100
12x + 10 + 6x = 100
18x = 90
x = 90/18 = 5