The question doesn't state the base of the logarithm, so I will initially assume it's base 10.
log10(8x+3)=64,
8x+3=1064,
8x=1064-3. 1064 is a very large number indeed and 3 is negligibly small so x is approximately 1064/8=(10/8)×1063=1.25×1063. Actual solution would be x=1,249,999...999.625. There are 61 nines in this number. Written out completely:
1,249,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999,999.625.