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

Rate_{1} / Rate_{2} = square root of (Mass_{2} / 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