You have a rectangle with length l = 4x - 3 and width w = -5x + 7
In order to know the perimeter of the rectangle you use the following formula:
P = 2w + 2l
where w and l are the width an length of the rectangle respectively.
By replacing the algebraic expressions for w and l into the previous equation for the perimeter you obtain:
P = 2(4x -3) + 2(-5x + 7) apply distribution property
P = 8x - 6 - 10x + 14
P = -2x + 8
P = 8 - 2x
Hence, the perimeter of the rectangle is 8 - 2x