My homework says name a 2 digit odd number that is prime. name a 2 digit odd number that is composite.

2 digit meaning that it has 2 numbers. For example, 66 is a two digit number.
Now, they also said it must be prime, meaning that its factors can only be 1 and itself. 
Also, it must be odd. 

 67

Now, do the same, except the number must be composite, as meaning that it has other factors. 

  21

a is 9 and b is 4

Step-by-step explanation:

When you subtract a-b, a is 9 and b is 4 which makes it 5. And ab, is 9 x 4 which equals 36.