Factorise x^2 + 2x – 15

What helps to factorise quadratic equations is to remember the rule that the number without a coefficient must be multiplied into. Since this is negative above, it means one of the factor terms must be negative. So one negative number and a positive number add to positive 2 and multiply to become negative 15. Only numbers that can give those answers are negative 3 and positive 5 giving us our answer. We can check it by multiplying the brackets out to see if we return to the original equation.