let the width be x, the length is then 3x-10
Area of a rectangle is length times width: x(3x-10)=77 => 3x^2-10x-77=0
Factor this quadratic equation: (x-7)(3x+11)=0 =.x=7 or x=-11/3
width cannot be negative, so the only answer is x=7
width is 7m, length is 11m
