Exponential decay can b expressed by the formula

y=a(1-b)^x,

wherein 'y' is the final amount, 'a' is the original amount, 'b' is the decay factor, and 'x' is the amount of time that has passed.

In your case

F(x)=0.9(0.7)^t

we have:

y = F(x)

a = 0.9

x = t

1-b = 0.7

If we solve '1-b = 0.7' for 'b' we get

b = 1 - 0.7

b = 0.3 that is 30%

The decay rate is 30%.