Ahot air balloon has a volume of 99 l at 15°c. to what temperature must be heated to raise its volume to 1200 l ,assuming the pressure remains constant? t =°c

Since the pressure is constant, you can use the equation V₁/T₁=V₂/T₂ and solve for T₂. (T₂=(T₁/V₁)V₂)
since this is a gas law type question, the temperatures need to be in kelvin therefore you need to convert 15°C to kelvin.  15°C+273=288K.  The volume are already in L so they don't have to be converted.  
now you just need to plug everything in:
T₂=(288K/99L)x1200L
T₂=3491K
The answer is you need if you have a 99L of gas at 288K, you have to increase the temperature to 3491K to get a volume of 1200L if the pressure remains constant.

I hope this helps.  Let me know in the comments if anything is unclear.