[Python] A more efficient solution for solving this matrix puzzle?

Find the 3x3 matrix, M, whose rows, columns and diagonals add up to 15. Condition: you must use every number from 1-9.

I'm not very clever, so I just tried this brute force method:

def solve_999():
for a in range(1, 10):
for b in range(1, 10):
for c in range(1, 10):
for d in range(1, 10):
for e in range(1, 10):
for f in range(1, 10):
for g in range(1, 10):
for h in range(1, 10):
for i in range(1, 10):
if (a+b+c == d+e+f == g+h+i == a+d+g == b+e+h == c+f+i == a+e+i == c+e+g == 15):
if check_unique([a, b, c, d, e, f, g, h, i]):
print(a, b, c)
print(d, e, f)
print(g, h, i)
return


def check_unique(L):
d = {1:0, 2:0, 3:0, 4:0, 5:0, 6:0, 7:0, 8:0, 9:0}
for letter in L:
d[letter] += 1
if d[letter] > 1:
return False
return True


It works, but it isn't very efficient. Can anyone help me find a more efficient solution?

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