Use parentheses, like this 1/(3 + i), next time so that we don't have to guess what you mean.
You need to multiply the fraction by an expression that is equal to 1, but that is in a form that has the conjugate of your denominator in the expression's denominator. If you have a complex number the conjugate of that complex number is . For this problem your conjugate is
An expression equal to 1 with in the denominator can only be:
Multiply this times your original expression:
Notice the difference of two squares factorization in the denominators and do the multiplication: