Get everything on one side:
. Now we can factor to find the solutions. What we ask ourselves here is "What 2 numbers will add to be +4 and multiply to be -21?" +7 and -3 work because 7-3 = 4 and 7 * -3 = -21. So our factors are (x+7)(x-3). Our solutions, however, are found using the Zero Product Property. If x + 7 = 0, then x = -7. If x - 3 = 0, then x = 3. So our solutions are x = -7 and x = 3.