It's rather sloppy not to use parentheses where they are needed... and frequently the result is that the solution you get is not to the problem you intended.
The standard rule is not to leave any imaginary parts in the denominator -- i.e., always answer the question in the form a+bi.
To clear an expression of the form a+bi in the denominator, you need to multiply numerator and denominator by the conjugate, a-bi. The product of a+bi and its conjugate is a^2+b^2; so the denominator will be a real number.
But in this example, before doing that, note that there is a common factor of 3 in both terms of both the numerator and denominator. So make your work easier by dividing out that common factor first.
Then when you multiply by the conjugate, you will have
