To find the sales tax as a percentage, we take the $147 item with its $14 sales tax. As a percentage $14 is 14/147%=9.523%. If we apply the same tax rate to $84 we get $8 exactly.
This is really a question on simple proportion. It's like saying: if $147 is equivalent to $14 (for any reason whatsoever) what is $84 equivalent to? To answer the question we just carry out the following arithmetic: 14/147*84=2/21*84=2*4=8. There's no need to be concerned about sales tax in this question, or wonder what it is.