the concentration of a mixture is guven be the following function:

c(t)=(2x+9)/3(x+12)
A.what is the maximum amount of concentration that this function can reach in terms of percent?

B. what is the name of the line you looke for to answer part A.

for positive values of x, this function is a slowly rising function which has a y-intercept of .25 and a limit of 2/3 as x approaches infinity

so we can say that the maximum concentration in terms of a percent is 66.67% (correct to 2 decimals)

B.
I don't quite understand your question.
Are you asking for the asymptote?

The tangent to this curve approaches the horizontal line y = 2/3