Using Henderson-Hasselbalch Equation to find volumes of acid and base component?

pH = pKa + log([base]/[acid])
3.74 = 3.74 + log (mole HCOO-)/(mole HCOOH)
mole HCOO-/mole HCOOH = 1

moles buffer = 0.100 mole/L x 0.100 L = 0.01 moles = moles HCOO- + moles HCOOH

mole HCOO-/mole HCOOH = 1
mole HCOO- = mole HCOOH

Again,
moles HCOO- + moles HCOOH = 0.01

Substitute

(moles HCOOH) + moles HCOOH = 0.01
2 (moles HCOOH) = 0.01
moles HCOOH = 0.005 moles HCOOH moles formic acid

Since moles HCOOH = moles HCOO-

moles HCOO- = 0.005 moles HCOO- moles formate ion

You didn't give the concentration of the stock solutions of HCOOH and HCOO- thus I cannot find the volume. If you have those values, you can continue from here. Remember that:

Molarity = moles solute / volume solution

That's it!n_n