what does an answer of {} mean?

when I type solve(a,b,ab=76,a+b=18), I get {} as an answer.

a b = 76..........(1)

a+b=18............(2)

From (2) a = 18 - b sub into (1)

b(18 -b) =76

18b - b^2 =76 re-arrange

-b^2 + 18b - 76 =0, solve for b

b =9 + sqrt(5), and b=9 - sqrt(5) sub into (2)

a + (9 + sqrt(5) =18

a = 9 - sqrt(5) and a =9 + sqrt(5)