(a) flow rate = velocity x cross section
Make sure units are consistant. If radius is in meters than flow rate should be in m^3 per second. Assuming units are consistant
3.8 x10^-9 = v x 2x10^-6
v = 1.9 x 10^-3 m/s make sure you check the units.
(b) Total blood flow = number of capillaries(n) x flow rate of one capillary
84 = n x 3.8 x 10^-9
n = 84/(3.8 x 10^-9) = 22 x 10^9