Question 27336
Start by taking out the biggest thing possible, which would be {{{9ab^3}}}.  
{{{(36/9)=4}}}, {{{((a^2)/a)=a}}}, {{{(45/9)=5}}}, and {{{((b^4)/(b^3))=b}}}, so you end up with {{{9ab^3(4a-5b)}}}.