Answer:
simplified: 

unsimplified: 

Explanation:
Since the fractions have the same denominator, all we have to do is subtract the numbers in the numerator:
15 - 3 = 12
The difference of the two fractions is 
. To simplify this answer, divide both the numerator and the denominator by 4:
12/4 = 3
20/4 = 5
The difference in simplified form is 
.
I hope this helps. :)