In the case of the first question when you are multiplying terms you simply add the exponents provided the variables are the same. In your case the x term will stay the same but you are able to combine the y terms. However the coefficients are multiplied together just like normal numbers.
The second question is delt with in the same way as the first.
For the final question
I'll let you work the other two questions out yourself because it's better for you to do them to make sure you understand. Hopefully I've explained well enough but if you need further help just ask. Feel free to post your answers and I'll check you've done the right thing if you like.