The other answer is slightly incorrect in dividing by 6x, since that assumes that x is not 0. Since we do not know this, we should avoid that. Instead try this:
Divide both sides of the equation by 6 so that it becomes
x^2 + 7x = 0
Now, use the quadratic formula to ensure you find both roots. The quadratic formula is given here:
http://www.purplemath.com/modules/quadform.htm
Applying the quadratic formula here, notice that a = 1, b = 7, and c = 0. Plugging these values into the quadratic formula, you get
x = (-7 +/- sqrt(7^2 - 4*1*0))/2
where +/- means "plus or minus".
Simplifying, you get
x = (-7 +/- 7)/2 = both (-7 + 7)/2 and (-7 - 7)/2
This means that x = 0 or x = -7. You can plug both of these back into the original formula to check that they are roots. So the answer x = -7 misses out on the fact that x = 0 is a root, and therefore you cannot divide out by 6x and recover this root successfully.