How many moles of HCl are present in 75.0mL of a 0.200M solution?

The concentration is 0.200 M which is 0.200 moles/Liter.

You have 75 mL. How many liters is this?

If you multiply Moles/Liter times Liters you'll get:

\frac {moles}{Liter}\cdot (Liter)=moles

The two "liter"s cancel out and you're lift with "moles"

So, figure out how many Liters you have if you have 75 milliliters and multiply that value times the concentration in moles per liter.