a certain gas takes 3 times as long to effuse out as helium.what will be its molecular mass?

You can use Graham's law of Effusion

Rate₁ / Rate₂ = square root of (Mass₂ / Mass ₁)

mass of helium = 4 units

 Rate of He  =  3 X Rate of unknown gas.

let the unknown gas has mass x

then

square root of (x / 4)  =  3

x / 4  = 9

x = 9 X 4  = 36 g/mol