4)
a) x^3m x^6 k^15 = x^21 k^15
x^(3m + 6) = x^21
3m+6 = 21
3m = 15
m = 5
b)
x^5 y^5 z^5 z^m = x^5 y^5 z
z^m+5 = z^1
m + 5 = 1
m = -4
c)
(x^8 / yz^5)^ m = 1
as you know any number with an exponent of zero is equal to 1
so answer
m = 0 to make the equation becomes true