Answer is: the missing pressure is 1088.66 mmHg.
Gay-Lussac's Law states that the pressure of a given amount of gas held at constant volume is directly proportional to the Kelvin temperature.
p₁/T₁ = p₂/T₂.
p₁ = 960 mmHg; pressure of the gas.
T₁ = 100°C + 273.15.
T₁ = 373.15 K; temperature of the gas.
T₂ = 150°C + 273.15.
T₂ = 423.15 K.
p₂ = p₁T₂/T₁.
p₂ = 960 mmHg · 423.15 K / 373.15 K.
p₂ = 1088.66 mmHg.
