Two parter question from a "fluid mechanics exam for second year students"

a)Derive a relation to find the critical pressure ratio in the case of isentropic flow.
b)If the velocity distribution over a plate is given by u=(2/3)y-y^2 in which u is the velocity in
meter per seconds at a distance y meter above the plate, determine the shear stress at y=0 and y=0.15m.
Take dynamic viscosity of fluid as 8.63 poises.

Could someone please explain this question to me in layman terms, I am a second year student in Thapar University Punjab and fluid mechanics is really not a strong subject of mine.. it would be nice if someone could explain it to me like i am 5..