is pretty much the same as before, so without much fuss
a = charges before tax on first city
b = charges before tax on second city
how much is 3.5% of a? (3.5/100) * a, 0.035a.
how much is 7.5% of b? (7.5/100) * b, 0.075b.
the taxes for both cities, summed up to 347.50
so, 0.035a + 0.75b = 347.50
we also know that, whatever "a" may be, the second city charged $500 less, so b = a - 500.

how much did the second city charge? well, b = a - 500.