How do I factorise this X^2 - 2x + 15= 0?

Perhaps you meant x^2 -2x -15 ? (x+3)(x-5) =0

Since there is no number in front of the x^2 term, you are looking for two numbers that mulitply to give you +15 and added together give you -2.

However, this does not appear possible. Did you mistype the equation? Maybe it was - 15?

x2 - 2x + 15 = 0
(x - 5)(x + 3) = 0
x = 5 or -3