Suppose a meter stick made of steel and one made of invar are the same length at 0°C. What is their difference in length at 39.0°C?
I would love to help you, but unless you give the coefficient of thermal expansion for invar, I do not want to risk you getting the problem wrong by using a different value from the one supplied to you.
Simply put, once you have this coefficient, you can plug it into the equation below:
Where △L is the change in length of the meter stick, A is the coefficient of linear thermal expansion, and L0 is the initial length, or 1 meter in this case.