In general, a complex fraction is normally formed of two fractions, one on top of the other (the numerator, denominator, or both contain a fraction). It is actually a ratio of two fractions.
To convert this to a simple fraction, rewrite as division:
You can also just multiply the top numerator by the bottom denominator and the top denominator by the bottom numerator:
If only the numerator or the denominator contains a fraction, rewrite the other one as a fraction, and then divide or multiply: