# Question #89cfc

Jul 26, 2017

$\left(a + 5\right) \left(a - 4\right)$

#### Explanation:

This is a quadratic equation because it has a power of 2 within it.

When factorising a quadratic of the form $a {x}^{2} + b x + c$, you can determine whether it factorises with integers (whole numbers) using the formula ${b}^{2} - 4 a c$. If this is a square number, it will factorise. In this case, ${b}^{2} - 4 a c = {1}^{2} - \left(4 \times 1 \times - 20\right)$ which equals 81 so it will factorise (${9}^{2} = 81$).

You need to find factors of $c \left(- 20\right)$ that differ by $b \left(1\right)$ which are $- 4 \mathmr{and} 5$.

You can double check this by multiplying out the brackets again to give the original equation.