Step-by-step explanation:
The formula for exponetial equation decay is
[tex]y = a(b) {}^{x} [/tex]
where a is the initial value and b is the decay factor.
A will be 56 since that our initial value.
b will be 0.9 since we are constantly decaying by 0.9.
so our equation is
[tex]y = 56(0.9) {}^{x} [/tex]