In a problem, you are given two pressures and one temperature at constant volume and amount of gas. you are asked to find a second temperature. what law should you use?

I don't know who it was named for but the law is

P1/T1 = P2/T2

Make sure the pressure units are the same (atmospheres or kPa usually) and that the temperature is in Degrees Kelvin which is derived from Celsius degrees.

Try Charles' Law for the name.

Gay-Lussac's Law.  

Step-by-step explanation:

The volume and number of moles are constant, so we can use Gay-Lussac’s Law:

At constant volume, the pressure exerted by a gas is directly proportional to its temperature.

p₁/T₁ = p₂/T₂     Invert each side of the equation

T₁/p₁ = T₂/p₂     Multiply each side by p₂

  T₂ = T₁ × p₁/p₂

  p₂ = p₁ × T₂/T₁

The units for the pressures don't matter if you use same units for each pressure.

However, the temperatures must be absolute values, usually measured in kelvins.