Factorise and solve X^2+8X+15=0

For any equation in the form ax^2+bx+c and a=1, the first step is to find a set of numbers. In this case, they must add up to 8 (b) and multiply to 15 (c). 

With trial and error, it can be found that these are +3 and +5.

Thus, factorise the equation to get (X+3)(X+5)=0.

We know from this that X+3=0 or X+5=0.

Hence, we can solve both equations to get X=-3 and X=-5.