A level binomial expansion

I’ve done the working out and I’ve gotten a different answer to the solution. What have I done wrong?
The answer in the solution says 18 and I’ve got 3.

The question:
Find the coefficient of x in the binomial expansion of (1+ 3x)^6. [1 mark]

I think I may know what I’ve done, is it 6C1 x 3^1?

Can't tell what you've done wrong, as you haven't posted any working.

I’ve worked it out .