21/7=3mins per wall with 5 people.
Because we assume that all of the people paint at the same rate, 3 X 5=15 mins for one person working alone.
15/3=5 mins per wall with 3 people X 10 people= 50 mins for 3 people to paint 10 walls.
The answer is 50 mins.
To get rid of

, you have to take the third root of both sides:
![\sqrt[3]{x^{3}} = \sqrt[3]{1}](https://tex.z-dn.net/?f=%20%5Csqrt%5B3%5D%7Bx%5E%7B3%7D%7D%20%3D%20%5Csqrt%5B3%5D%7B1%7D%20)
But that won't help you with understanding the problem. It is better to write

as a product of 2 polynomials:

From this we know, that

is the solution. Another solutions (complex roots) are the roots of quadratic equation.
We need to make h alone.
Add 7 to both sides to remove -7 from the right, and multiply both sides by -1 to make h positive. Doesn't matter which you do first.
Result:
6=h
h=6