Now this part I'm not typing out all the steps because its annoying. But you have the radius but you need the height. So you solve this:
h = 100/(pi*r^2)
h = (100/pi)/((50/pi)^(1/3))2

Take my word for it it breaks down into:
h = 2 * (50/∏)^(1/3)
h = 2 * r
h = 2r

Then plug it all back into the original and simplify:
M = 2*pi*r*h + 2*pi*r^2
M = 4*pi*r^2 + 2*pi*r^2
M = 6*pi*r^2
M = 6pi[(50/∏)^(1/3)]2

And thats it.

Enjoy doing your second problem.