That, because pressure and temperature have a direct relationship and pressure and volume have an indirect relationship, crushing (decreasing the volume) and adding heat (increasing the temperature) will cause the aerosol can to exceed its pressure limitation and may explode.
We should convert any of our parameters to units that would be easy to use with a known R value. One option would be to have the pressure in atm, volume in L, and convert the mass of the hydrogen gas to moles:
P = 1.2 atm V = 750 mL = 0.750 L n = (0.30 g H2)/(2.0159 g/mol) = 0.1488 mol H2 R = 0.0821 L•atm/mol•K.
Solving for T,
T = (1.2 atm)(0.750 L)/(0.1488 mol H2)(0.0821 L•atm/mol•K) = 73.671 K.
If you opt to leave your temperature in Kelvin, I would go with 74 K, as that has two significant figures like the rest of the values.
If you opt to go with Celsius, then the answer would be a bit awkward if you had to follow two sig figs: 73.671 - 273.15 = -199.49 ≈ -2.0 × 10² °C.