Step-by-step explanation:
An exponential function has the form:
y = a · b^x
Are you sure you copied the problem correctly? Perhaps instead of minus, you meant multiplication:
f(x) = 620 · 7.8^x
If so, then a = 620 and b = 7.8.
4.0 rounded to the nearest tenth
To find the decreased percentage,here's what we can do:

In this case,the eqaution would be:

Hope it helps!