Determine the degree of the following:
a. - 7x ^ 2

b. 3x ^ 3 - 5x ^ 4 + 9x ^ 2 - x

c. a ^ 3 * b ^ 2 + 3ac - 4a ^ 2 * b ^ 4

d. -9

e. 4x * y ^ 2 * z ^ 3​