Call the width x. Note that the length is 3x-6. We know that double the width plus double the length is 120, so we write an equation:
\(2(x)+2(3x-6)=120\)
\(2x+6x-12=120\)
\(8x-12=120\)
\(8x=132\)
\(x=16.5\)
If x is 16.5, the width is 16.5, and the length is \(3 \cdot 16.5 - 6 = 43.5\).
