Question

factorise c²-4cd-21d²​

Answer 1

$\text{The given equation is }c^{2} - 4cd - 21d^{2}$

$\implies \text{We can write }(-4cd) \text{ as }(-7cd + 3cd)$

$\implies \text{The equation becomes }c^{2} - 7cd + 3cd - 21d^{2}$

$\implies c(c-7d)+3d(c-7d)$

$\implies \boxed{(c-7d)(c+3d)}$