Suppose you have a product a*b*c = 0. Then if any of a, b or c is equal to zero, the equation is true. This is because any number times zero is equal to zero. For example, if you have a = 0, then 0*b*c = 0, which is true. This is true for any number of terms in the product.
In your case, you have two terms in your product, (3k)*(k+10) = 0. As per above, the equation is true when any of the terms are zero. We solve for this algebraically by setting each one to zero.
3k = 0 -> k = 0
k + 10 = 0 -> k = -10
These are the solutions to the problem.
