You MUST have SOME idea. You simply MUST. If nothing else, you could try the hint.

i = 0.05 -- This is annual interest

j = i/12 = 0.00416666... -- This is monthly interest

v = 1/(1+j) = 0.995850622406639 -- This is the monthly discount factor

That's all you need. Now build it.

300000 = M*(v + v^2 + v^3 + ... + v^360)

Can you add that up inside the parentheses?

Note: This is a geometric series.