for above question I think I've gone about it correctly, just looking for a bit of verification or any pointers if I've made an error.

Here goes: (SORRY, NOT SURE WHY THE Pi SYMBOL HAS PASTE INTO THAT LITTLE SQUARE THING)

y = 6000 cos(t/6 - /4)

dy/dt = d/dt[6000 cos(t/6 - /4)]

= 6000*d/dt[cos(t/6 - /4)]

= 6000* sin(t/6 - /4)* d/dt(t/6 - /4)

= 6000* sin(t/6 - /4)* /6

= 6000/6 * sin(t/6 - /4)

= 1000 sin(t/6 - /4)

Flow = = 1000 sin(t/6 - /4)

NOTE:

d/dt(t/6 - /4) = d/dt(t/6) - d/dt( /4)

= /6*d/dt(t) – 0 since derivative of constant is 0

= /6 *1

= /6

THANKS!