we can use the clausius clapeyron equation to solve this

.. ln(P1 / P2) = (dHvap / R) x (1/T2 - 1/T1)

note we're given P1, P2, T1, T2 and P3 and asked to find T3.

so if we assume dHvap and R are constants, we can write

.. ln(P1 / P2) = (dHvap / R) x (1/T2 - 1/T1)

.. ln(P1 / P3) = (dHvap / R) x (1/T3 - 1/T1)

now.. you can manipulate those equations and cancel out dHvap/R to generate

1 equation in terms of P1, P2, P3, T1, T2, and T3.. . or if you prefer, you could

just solve for dHvap.R in the first equation, and carry that over to the 2nd.

let's do that

.. (dHvap/R) = ln(P1 / 2P1) / (1/375.15 - 1/264.15) =

.. ... ... .. ... .= ln(1 / 2) / (1/375.15K - 1/264.15K)

.. .... ... ... ...= 8608.3 K

plugging that into the 2nd

.. 1/T3 = 1/T1 + ln(P1 / P3) / (dHvap/R)

.. 1/T3 = 1/364.15K + ln(1/6) / (8608.3K)

.. . ..T3 = 394.0K = 121°C