You may want to review imaginary numbers first.
You can add or subtract multiples of i with real numbers, and then it's called a complex number. (Technically, the real and imaginary numbers are also special kinds of complex number; so, if you're asked, “Is this number complex?” the answer's almost certain to be yes.) Suppose you're given the equation x − 3i = 1 + yi. If you know that x and y are both real numbers, then you know what x and y are; x = 1 and y = −3.