Don't worry: if you do, the paper won't burst into flames. In other words, why don't you try?
ar^{7} = 360 and ar^{4} = 45. Divide the first equation by the second one, i.e., divide the left-hand sides and then the right-hand sides and equate the results. Alternatively, express a from the second equation: a = 45 / r^{4} and substitute it into the first equation.