Okay, here we have this:
Considering the provided information, we are going to find the requested number "k", so we obtain the following:
Since the y-intercept is the value of y when x equals zero then we get:
12x + ky + 20 = 0
12(0)+k(-9)+20=0
-9k+20=0
-9k=-20
k=-20/-9
k=20/9
Finally we obtain that k is equal to 20/9.