Making you do the last step to make sure you understand it. Make sure you do a similar problem in your spare time.
For the second problem. You need to work out the mass of each of the components and then add them up. Once you have done this, divide by the total volume to get density.
Multiply the volume by the density to get the mass.
If you need any more help, feel free to post. Same goes if you are confused about what I have done.