Answer
$-91.94$ degrees X.
Work Step by Step
Let's find a linear conversion from X to Kelvin. Saying $-170a+b = 27$3 and $-53.5a+b = 373$, we find that $a = 0.8584$ and $b = 418.9227$. Setting $0.8584x+418.9227 = 340$, we find that $x = -91.94$ degrees X.