A hot-air balloon has a volume of 970 L at 28 degrees C . To what temperature must it be heated to raise its volume to 1200 L , assuming the pressure remains constant?

so it would be 970L/28degreesC=1200L/x? I did that and I got 34.6 degrees C and when i put it into my answer slot on MasteringChemistry it says its wrong.

Am I doing something wrong?

NO. Temps are in Kelvins, not C, in that equation. Convert to K.