Question 175212
Area equals length times width.  Since the area is {{{x^2+50x}}} and the width is {{{x}}}, we know that {{{x}}} times something representing the length must equal {{{x^2+50x}}}.


Notice that if you factor an {{{x}}} out of {{{x^2+50x}}}, you get:


{{{x^2+50x=x(x+50)}}}.


Since {{{x}}} is the width, {{{x+50}}} must be the length.