First off, nicely done on the exponents. You'd be surprised on how bad some look ,:D
a)
[tex] (3xy^{3})(2x^{2}y)\\
(3*2)(x*x^{2})(y^{3}*y)\\\\
6x^{3}y^{4} [/tex]
b)
[tex] 2y(2y+3y^{2})-y^{2}(3-y)\\
(4y^{2}+6y^{3}) + (-3y^{2}+y^{3})\\
(6y^{3}+y^{3})+(4y^{2}-3y^{2})\\
7y^{3}+y^{2}\\\\
y^{2}(7y+1) [/tex]
