you know a = dv/dt . it can also be written as dv/dx * dx/dt. where x is the distance. so your dx/dt is nothing but v. To sum up. a = v*dv/dx = -ksqurt(v). Now you can integrate it . the limits for x are 0 to 18. and v are 81 to 36. becareful with the signs. after ur done . you get the value of k. and also u get the equation of velocity vrs distance.
a)substitute x = 24 , in the quation and u will get the velocity.
b) you know velocity = dx/dt. intergrate it , u will get the time.