-9 in power means that the value is in fractional and the 10 base is multipied 9 times and put it on denominator.
8.6 = 86 x 10^-1
Let's multiply :
86 x 10^-1 x 10^-9
= 86 x 10^-10
= 86/10000000000
=0,0000000086
Note : if you don't know about how 10^-1 x 10^-9 returns 10^-10, it's because of exponential rules. If the base are same wuth each other, then just do the addition operation in the exponent like this :
10^(-1+(-9))
= 10^(-1-9)
=10^-10
Hope it helps :)