Question 274690: A rectangular box has dimensions 4ft by 5ft by 5ft. Increasing each dimension of the box by the same amount yields a new box with volume seven times the old. Find how much each dimension of the orginal box was increased to create the new box. Round your answer to two decimal places.
This is where I started:
0= x^3+20x^2 ---->this part isn't finished, I don't understand how to get it...please help.