7 L of an acid solution was mixed with 3 L of a 15% acid solution to make a 29% acid solution. What is the percent concentration of the first solution?

well, let's say that the first solution is 7 liters and has "x" percent of acid, how much acid total in it? well, 7x, keeping in mind that "x" is a decimal form.

likewise, the second solution is 3 liters and has 15% acid, so how much acid is there in that one?  (15/100) * 3 = 0.45, so

f(x) has value 150 at x= 0

and 0 at x= 3

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