Please note that the concentration og HI is [HI]= 0.43/1 = 0.43 mol/liter, so the concentration of H2, is [H2]=0.215 mol/liter, since we have 0.43/2 = 0.215 mol of H2. Now at the Equilibrium we have the subsequent concentrations:

[H2] = y

[I2]= y

[HI]= 0.43- 2y

Next I substitute all those quantities into the expression for the Equilibrium constant, and I get this:

58 * 10^(-4) =[ (0.43 -2y)^2]/ (y^2), and taking the square roots of both sides I can write: