6x(to the power of 2) + 42x=0
Factor out a$\displaystyle 6x$.
$\displaystyle \begin{aligned} 6x^2 + 42x & = 0 \\ 6x(x + 7) & = 0 \end{aligned}$
Now note that if two numbers multiplied together equals to 0, then either one or both of the numbers are 0. So let's set both factors (6x and the x + 7) equal to 0 to see what we get.
$\displaystyle 6x = 0 \qquad x + 7 = 0$
Can you carry on from here?