Arectangle has a perimeter of 8x+6 and a length of 3x. what is the width

let A and B be the two sides of the rectangle.

The perimeter is the sum of its four sides:

perimeter = A + B + A + B = 2A + 2B = 8x + 6

We are also given one of its sides (length), say, A = 3x

and so we know how to get the second side (width) B:

2A + 2B = 8x + 6

6x + 2B = 8x + 6

2B = 2x + 6

B = x + 3

The width of the rectangle is x + 3