To get this answer you would use the formula A = lw to find the missing dimension.
You will substitite in what you know the area is and an expression for the length.
154 = w (3 + w)
154 = 3w + w^2
Create an equivalent equation equal to 0.
w ^2 + 3w -154 =0
(w - 11 ) (w + 14) = 0
w would be either 11 or -14 for this to be true.
Only 11 is a possible dimension, so the width is 11 meters.