3x + 2y = 12
y = x - 9

y=12/2

y=6

x=12/3

x=4

y= x-9

Y=4-9

y=5

6=5

6/5

1.2

f(x) has value 150 at x= 0

and 0 at x= 3

so d(f(x))/dt = slope = -150/3 = -50

i think that you should ask google for the answer to that problem because if we don't know it google does