Ok, guessing this is fluid dynamics? Not really my subject but will give it a go.
i: Area of a square duct is b^2, area of a circle is pixr^2, so d = 2r, r =root( b^2/pi) Then work from there?
ii: There should be an equation for the flow between parallel plates, we know its square so we have a duct bxb.
iii: from part 2 we have an answer for a bxb... applying the same equation to an axb set of parallel plates should work.
(y) = (y*y - L*L) * 1/2 * 1/visc * dp/dx (where 2*L=plate separation, p=pressure)
Columbia Uni