First, you should convert the temperature unit to absolute temperature.
Second, you shoul graph the points. Then you will find a pretty linear correlations among the points.
You can pick between using the best fit line or you could observe that as you get to higher temperatures the linear behavior is "more perfect".
I found this best fit line:
P = 2.608T + 14 Then, for T = 423K
P = 2.608(423) + 14 = 1117 mmHg
If you prefer to use the last two points, this is the calculus:
[P - P1] / [T - T1] = [P2 - P1] / [T2 - T1]
[P - 960]/[423 -373] = [960 - 880] / [373- 343]
=> P = 1093.3 mmHg.
You can pick any of the results 1177 mmHg or 1093 mmHg, You need more insight to choose one of them: conditions and error of the experiment for example.
