I'll walk you through the steps you'll need to carry out, and you can do the actual calculations. I'm going to assume that you want to calculate R with units of L atm / mol K. If you are to calculate R with other units, you'll just do other conversions of the values and information that you have.
So, you can rearrange the ideal gas law to the form:
PV/nT = R
In order to calculate R, you need a pressure (you have in in mm Hg, so convert that to atm).
You need a temperature, which you have. Just convert that to Kelvin.
You have a volume in ET unit, so you can convert that to mL and then to L.
41.90 Et Units X 0.466 mL / ET unit X (1 L/1000 mL) = 0.0195 L
The only value you don't immediately have is the moles of H2. But you have a mass of Mg. Using the mass of Mg and the balanced reaction, you can calculate moles of H2 formed:
Mg + 2 HCl --> MgCl2 + H2(g)
0.04370 g Mg / 24.3305 g/mol X (1 mol H2 / mol Mg) = 1.298X10^-3 mol H2
Now, it's just a matter of plugging into the ideal gas law and calculating R.