Calculating the initial concentration of H2 if no HI was present?

Question

Ok i did the first part which was finding out the HI, but i cant think how to do the later of this problem, the finding initial concentration.::::




The equilibrium constant for the gas phase reaction H2 + I2<=>2HI is 54.8. The equilibrium concentration of HI is 1.735740303 and the equilibrium concentrations of H2 and I2 are
0.153 and 0.357 mol L-1 respectively.

Calculate the initial concentration of H2 if no H2 was present at
the start of the reaction

Answer 1

Since all of the HI had to come from the reaction of H2 and I2, then the initial concentration of H2 would be equal to its equilibrium concentration + 1/2 of the HI concentration at equilibrium, or 0.153 + 1.736/2 = 1.021.