We know that the perimeter is 42 and the length is 2.5 the width, so we can create a system of equations
2l+2w=42
l=2.5w
Substitute l into the equation.
2(2.5w)+2w=42
5w+2w=42
7w=42
w=6
To solve for l, plug w into the second equation.
l=2.5(6)
l=15
Therefore, the width of the rectangle is 6 and the length of the rectangle is 15.
I hope this helped :)
