Factorise:  $49x^2-28xy+4y^2$

We know that, $a^2-2ab+b^2 = {(a-b)}^2$ 

$49x^2-28xy+4y^2$ can also be written as ${\left(7x\right)}^2-2\left(7x\right)\left(2y\right)+{\left(2y\right)}^2$ 

Here, a = 7x and b = 2y

$\therefore 49x^2-28xy+4y^2$ = ${\left(7x\right)}^2-2\left(7x\right)\left(2y\right)+{\left(2y\right)}^2$ 

= ${(7x-2y)}^2$