One of the terms in the expansion of (x+3)^7 is kx^5. How do I determine the value of k.

termr+1 = C(7,r)x^(7-r) (3)^r

comparing the terms, 7-r = 5, so r = 2
so term3 = (7,2)x^5(3)^2
= 21(9)x^5 = 189x^5