The question is the picture

given, 4/12=x/18

now,

4/12=x/18

or, 18×4=12x

or, 72 =12x

or, x=72/12

therefore, x=6

so, the answer is x=6

f(x) has value 150 at x= 0

and 0 at x= 3

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