To be more explicit, you have two equations already and now Ygggdrasil has given you a third. You can combine these to find the answer you're seeking. When trying to solve something with equations it's helpful to write down what you know and what you are seeking. It helps clarify the process.
Known
[tex]Density = \frac{0.775 g}{mL}[/tex]
[tex]Volume = 50.0 mL[/tex]
[tex]Density = \frac{Mass}{Volume}[/tex]
Seeking
The Mass
Method
Look at the equations you have and try to rearrange them to isolate the thing you want (mass) on one side of an equation. Do this without numbers at first. Just look at the variables first (density, mass, and volume).
1) Isolate the variable you are seeking (mass) using algebra
2) Plug in the values you have to find the answer
What do you get?