Factorise:

The given expression is $y^{2}-21 y+90$.

Find two numbers that follow the conditions given below:

Sum $=-21$

Product $=90$

Clearly, the numbers are $-15$ and $-6$.

$y^{2}-21 y+90=y^{2}-15 y-6 y+90$

$=y(y-15)-6(y-15)$

$=(y-15)(y-6)$