Sheldon would have $21,386, while Howard would have $20,073.
The equation for each of these will be in the form

,
where A is the total amount in the account, p is the principal invested, r is the interest rate expressed as a decimal number, and t is the amount of time.
For Sheldon:
A=15000(1+0.03)¹²=15000(1.03)¹²=21386.41≈21386
For Howard:
A=15000(1+0.06)⁵=15000(1.06)⁵=20073.38≈20073