Answer
$T = 1800~K$
Work Step by Step
In part (a), we found that there are $1.5$ moles of gas.
We can find the temperature at point $b$:
$PV = nRT$
$T = \frac{PV}{nR}$
$T = \frac{(7500~Pa)(3.0~m^3)}{(1.5~mol)(8.31~J/mol\cdot K)}$
$T = 1800~K$