python中2个元组怎么想减

a=[1,2,3,4],[4,5,6,7]）

b=([1,2,3,4],[4,5,6,7],[6,7,8,9])

怎么样让 c=b-a

使c=([6,7,8,9])

a=([1,2,3,4],[4,5,6,7])
b=([1,2,3,4],[4,5,6,7],[6,7,8,9])
c = []
for j in b:
    if j not in a:
        c.append(j)
c = tuple(c)
print(c)