As given the two quantities given are pressure units
the relation between psi and in Hg is
1 psi = 2.04 inch mercury
Or
0.491 psi = 1 inch mercury
the problem can be solved by unit factor label method
the unit factor for the two units will be

We have to convert 43 in Hg to psi so we will use following conversion factor
43 in Hg = 43 X 0.491 psi = 21.11 psi pressure