This is biological application question, so the limit can't be negative infinity, which is what I would use as my answer for a regular question. The rate at which substance is eliminated from the body is proportional to the amount of substance present, and satisfies the differentail equation dx/dt=a-bx, where a and b are positive constants. What is the limiting concentration of substance(limit as t goes to infinity of x(t))? Can't integrate for this because it's a course on differential calculus only. Any pointers?