Hello, how do I write algebraic fractions using symbols?

I HAVE BEEN TRYING TO POST A THREAD BUT THERE HAVE BEEN NO BREAK THROUGH:

Simplify

/frac{m+n}{2}-frac{m-n}{3} + 3(m-1)

The expression should read m+n over 2 minus m-n over 3 plus 3(m-1)

I need help please.