You got 3x2 + 2x = -15 or 3x2 + 2x + 15 = 0. At this point I would use the general quadratic equation which is that
if ax2 + bx + c = 0 then
which in your case gives
This is called a complex number, numbers that involve the square roots of negative numbers. Your daughter might be familiar with these or it may be that the teacher meant y = -3 which gives real number answers as opposed to complex numbers. In that case 3x2 + 2x - 21 = 0 factors as (3x - 7)(x+3) = 0, which yields the two solutions x = 7/3 and x = -3.
Penny and Claude