Answer
$$ T_1= \frac{P_1T_2}{P_2} .$$
Work Step by Step
We multiply both sides by $T_1$ and then we divide by $P_2$ as follows:
$$P_2T_1=P_1T_2 \Longrightarrow \frac{P_2T_1}{P_2}= \frac{P_1T_2}{P_2} \Longrightarrow T_1= \frac{P_1T_2}{P_2} .$$