Question

Worldwide energy use in all forms increased linearly from the equivalent of 3.5 billion tons of oil in 1970 to the equivalent of 6.5 billion tons of oil in 2000
a) Express the energy use as a function of the year in the form y=mx+b where x is the year and y is the energy use is billions of tons of oil.
b) Use the function from part (a) to predict the world-wide energy use in 2010.

I'm having trouble figuring out the equation.