4.0 l sample of methane gas is collected at 30.0c,
what is the volume of the sample at -8c?

This particular law is a gas law, called Charle's Law. The formula is:

V1    V2
=
T1     T2

So we know our original volume is 4.0L, so we would plug that into our V1. We know T1 is the 30 degrees, since it relates to our original volume. However, we need to convert it to kelvin. We do this simply by adding 273 degrees to the 30 degrees, since 273 is the constant for kelvin.

We do not know our second volume, however we know out T2. It is -8 degrees, and don't forget to convert it to Kelvin. 

So, when we plug all of these numbers into the equation, we are left with V2 to find. To do this we cross multiply (V1 x T2) and then divide by T1. That leaves us with the number for V2. Don't forget to round to the least # of sig figs! And you can divide V1 by T1, and then divide V2 by T2, to ensure your answers are the same, since they are directly porportional and need to be equal to each other. 

Hope I could help!