Answer
$32400ab^4$
Work Step by Step
According to the Binomial Theorem we can obtain the $r+1$th term of the expansion of the binomial $(x+y)^n$ by the formula $_nC_rx^{n-r}y^r$.
Hence here it is: (by plugging in $n=5,r=4$ and $5a,6b$): $_{5}C_4(5a)^{5-4}(6b)^4=5\cdot5a\cdot1296b^4=32400ab^4$
