Question

Factorise:
x² – 21x + 90

Answer 1

$$\begin{gathered} x^2-21x+90 \\ =x^2-15x-6x+90 \\ =x\left(x-15\right)-6\left(x-15\right) \\ =\left(x-6\right)\left(x-15\right) \end{gathered}$$