9i/1-3i, i know you have to mult. by conjugate i just don't know how to mult. the bottom.

You get the conjugate of a binomial (two terms) by multiplying the second term by -1.
For example, the conjugate of x + y is x - y. In other words I change the sign of the second term.

The conjugate is useful because when I multiply a binomial and its conjugate, for example (x + y)(x - y), I get . The middle two terms xy and -xy cancel.
You may recognize the final result  as a difference of squares, in which the factored form is (x + y)(x - y).

The conjugate is especially helpful for simplifying fractions with imaginary numbers i (i is the square root of -1) because when the second term of a binomial has an i, you can multiply the binomial by the conjugate, in which the i will be squared, and , and of course -1 is more simplified and easier to deal with than i.