first we writte the information they are giving to us:
For company A:
initial fee = $90
rent per mile = $2
For combany B:
initial fee = $50
rent per mile = $3
Now for company A the equation that represent the charge in one mile will be: (where n is the number of miles)

And for company B:

Now to determine the number miles driven, that would make the cost of each company the same, we have to make A = B

and finaly we can solve for n

so if they drive for 40 miles the will pay the same