Originally Posted by
TheEmptySet Use the binomial Theorem
$\displaystyle (x+y)^n=\sum_{k=0}^{n}\binom{n}{k}x^{n-k}y^{k}$
Since the binomial theorem starts at k=0 the fifth term will be when k=4
$\displaystyle \binom{5}{4}(5a)^{5-4}(6b)^4=$
Just simplify from here.