If the function y = (a + 3) x + a2-9 is a positive proportional function, then a=______ .

If the function y = (a + 3) x + a2-9 is a positive proportional function, then a=______ .

∵ function y = (a + 3) x + a2-9 is a positive proportional function, ∵ a2-9 = 0, a + 3 ≠ 0, the solution is: a = 3