1. Factorisation

9at² - 2b + 18bt² - a

factorise it.

2. Originally Posted by helloying
9at² - 2b + 18bt² - a

factorise it.
Group the parts containing t :
$\displaystyle =(9at^2+18bt^2)-(a+2b)$

factor the left part :
$\displaystyle =9t^2 (a+2b)-(a+2b)$

$\displaystyle =(a+2b)(9t^2-1)$

use the difference of two squares, because $\displaystyle 9t^2=(3t)^2$ :
$\displaystyle =(a+2b)(3t-1)(3t+1)$