y=3(10)x as the littlle number at the close of the brackets

Is this the problem?

y=3\times\10^x

And what are you trying to solve? Are you trying to solve for "x"?

log(y)=log(3\times\10^x) =log(3) + log(10^x)=log(3)+x\,log(10)

x = \frac {log(y)\,-log(3)}{log(10)}

There's a little problem of communication here... you just mentioned exponential decay, and you give what seem to be

y=3\times10^x

Decay points out that the curve decreases, like in

y=3\times10^{-x}