How do you do this question? if you have 35.00mL of 0.300M HCl, how many moles of HCl do you have

You can think of the definition of molarity as an equation:
Molarity = moles of solute / liters of solution
M = mol / L
Then solve for moles:
mol = M x L
mol = 0.300 mol/L x 0.03500L
mol = 0.0105 mol

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You can use the unit-factor method and the appropriate conversion factor, which in this case is the molarity. The molarity is a ratio of two quantities, both referring to the solution.

0.03500L x 0.300mol HCl / 1 L = 0.0105 mol HCl

n(number of moles)=C(concentration in mol/L) x V (volume in litres)
=0.300M x 0.03500L
=0.0105mol HCl

Concentration=No of moles/volume
So your concentration=0.3 M=0.3mol/L and volume=35mL==> 0.035L

So n=concentration x volume
n=0.3mol/L x 0.035L==>0.0105 mol