Solve quadratic equation whose roots are 2,5????

Study this page until it seems obvious:
en.wikipedia.org/wiki/Polynomial_remainder_theorem

(x - 2)(x - 5) = 0

A big part of math is pattern recognition. A lot of homework is just fighting with stuff so you will remember the pattern when you see it again. The most common pattern is (x + a) * (x + b) = x^2 + (a + b)x + ab and the special case (x + a) * (x - a) = x^2 - a^2. When you spot the pattern you can just write the answer from memory.