math/trigonometry
posted by Sally .
solve this equation by completing the square.
x^2+4x21=0

halve the middle term, then square it, then balance the equation by adding it to one end of the = side and subtracting it from the other. Then get everything over to the left side so you have 0 left again on the right side, simplify, and factor. After factoring, take the two bubbles and set each equal to zero. This is how you get two numbers. Check your answers to make sure both of them work, because sometimes only one works.