There's a formula for that sort of thing, but feel free to build it from basic principles.

n = 18 years = 18*12 months = 216 months

i = 0.06

i^{(12)} = 0.06/12 = 0.005

Monthly Accumulation Factor: a = 1+i^{(12)} = 1.005

Monthly Deposit: P <== This is what we seek.

Final Accumulation: A = $120,000

If you had to do it in one payment, what would it look like?

P*a = 120000 and P = 120000/1.005 = 119402.99

If you had to do it in two payments, what would it look like?

= P(1.010025 + 1.005) = P(2.015025) and P = 120000/2.015025 = 59.552.62

It is hoped that you can see into the past and answer this last question, what would it look like if you have 216 months?

Your task is to add up all that stuff in the parentheses. It is a finite geometric series. Can you finish?

I get $308.26, so perhaps we have not defined the problem correctly.