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.