The fourth term in an arithmetic sequence is 18 and the seventh term is 39. If the first term is a, which is an equation for the nth term of this sequence?

Isabelle is mixing red and blue paint to make purple paint she adds 3/10 of a fluid ounce of red to 12/25 of a fluid ounce of blue to make 1 1/30 fluid ounces of purple how many fluid ounces of red paint ounces of red paint will she need to make 3 fluid ounces of purple paint