A source and an observer are each traveling at 0.50 times the speed of sound. The source

Question:

A source and an observer are each traveling at 0.50 times the speed of sound. The source emits sound waves at 1.0 kHz. Find the observed frequency if
(a) the source and observer are moving toward each other;
(b) the source and observer are moving away from each other;
(c) the source and observer are moving in the same direction?