why MU of the commodity declines demand for the commodity must be rising consequent upon a fall in its price explain it with reference to consumers equilibrium equation for a single commodity(p. 56) ?

Dear student,
According to the Law of Demand, price and demand share a negative relationship, i.e. if price falls, demand increases, and vice-versa.
and according to the Law of Diminishing Marginal Utility, as more and more units of a commodity are consumed, the marginal utility derived from it falls. This is because, with each additional consumption, your satisfaction derived from consuming that unit falls.
So, we can say that in case price falls which in turn increases the demand, the marginal utility of that commodity will fall as now the consumer is demanding more and thus, consuming more units of that commodity.
Moreover, the equilibrium condition in case of a single commodity is, when the Marginal utility of a rupee spent on a commodity is equal to the Marginal utility of Money.
$\frac{M{U}_x}{P_x}=M{U}_M$
So, in case the price falls, the Marginal utility for that good will also fall as a result of which the marginal utility of rupee spent on that commodity will decrease.
I hope this clears your doubt. In case of any other doubts, feel free to ask!