$[12]$ Consider the following reaction:

$3Mn{O}_4^{2-}(aq)+4H^{+}(aq)2Mn{O}_4^{-}(aq)+Mn{O}_2(s)+2H_{2}O(I)$

If the rate of disappearance of the manganate ion $(Mn{O}_4^{2-})$ is $3\mathrm{.}3{10}^{-2}\frac{M}{s},$ calculate:

a$.$ The rate of disappearance of the hydronium ion. $[3]$

b$.$ The rate of appearance of $Mn{O}_4^{-}.[3]$

c$.\frac{[Mn{O}_2]}{t}$

d$.$ The average rate of the reaction. $[3]$