OpenSAT

We can solve this quadratic equation by factoring. First, we need to move all the terms to one side: $x^2 + 4x - 21 = 0$. Then we factor the quadratic expression: $(x+7)(x-3) = 0$. For the product of two terms to be zero, at least one of the terms must be zero. Therefore, either $x+7 = 0$ or $x-3 = 0$, which gives us $x = -7$ or $x=3$.