A typical exponential function is y=a

when x=3.5, y=16.2
when x=6, y=3936.6
plug these values into the exponential function:
16.2=a

3936.6=a

divide the second equation by the first to eliminate a:
243=

log both sides: log243=2.5logb
logb=log243/2.5
use your calculator to find b: b=3.6
plug b=3.6 in the first equation to find a:
16.2=a

a=0.183
please double check my calculation