Finding the zeros is just factoring the equation. It is also where the parabola crosses the x- axis.
First you need to find two numbers that mutliply to give you -21 and add to give you 4 then you put it into this form:
(x +7)(x-3) set each part equal to zero
x+7=0 and x-3=0 you get:
x=-7 and x=3 your zeros