The easy way is to look at the correlation of the numbers. 3 is half of 6. So the answer must be a, 36 and 72 because 36 is half of 72.
So (36,72).
Now the right way is to set up an equation.
The equation is

Find slope


So

Now plug in a point to solve for b

So the equation is

Now lets check every answer. Note that the first one works.
So the first one is the right answer.