Question 987581
You are on the rightrtrack. Just made a mistake on the length.

You have the width as x, and the length as 3x. The problem said the length was 3cm greater. Not 3 times greater. Use a length of (x+3) and then solve using the same techniques

{{{40 = x*(x+3)}}}

{{{40 = x^2 + 3x}}}
{{{0 = x^2 + 3x - 40}}}
{{{0 = (x+8)(x-5)}}}

you can take in in from there