h is the plank constant

V_f - is the Fermi velocity in graphene (10^6m/s)

the square-root thing is basically Fermi wave vector calculated based on the charge carrier surface density for a given gate voltage V_Si. h_a +h_b should add up to the distance between the graphene sheet and the gate electrode (silicon wafer). The charge carrier surface density is V_si * C_s / q_e with the C_s being the unit area capacitance and q_e - electron charge