solve the inequality x^2+4x-21>0

First we should try and factorise the polynomial the brackets will be (x+?) and (x-?) as the coefficient of x and the -21 have differing signs.just thinking of the factors of 21 we can see that 7-3 = 4 the factors are therefore (x-3)(x+4)the graph of (x-3)(x+7)=y crosses the x axis at x=3 and x= -7from this we can see that x < -7 or x > 3