combine like terms ( terms with the same variable and are to the same power) so thats 5b² and -3b² and 2a and -a. the -b doesnt have any like terms so leave that alone for now. combine like terms 5b²-3b² is 2b² and 2a-a is a. so you have 2b²+a and plug in the -b so you get 2b²+a-b.
