Factorise and solve x^2 - 8x + 15 = 0.

The easiest way to solve this problems it via trial and error. For any quatdratic (ax² + bx  + C = 0), the coeficcent of x, (in this case b = 8) is the sum(+) of the two solutions and c is the product(). So you need to find two numbers that add to give -8 and multiply to give 15. In this case it is -3 and -5 as -3+-5=-8 and -3-5=15. This measn the answer is (x-3)(x-5) = 0, so x= 3, or 5.