6% tax means 6% of $3.50
Convert 6% to decimal (move decimal 2 spots to the left): 6% ⇒ .06
Multiply 3.50 with .06 = .21 <-- That's .21 cents of 3.50
That means you have to pay an extra 21 cents. Add that to the $3.50 you had to pay.
$3.50 + $0.21 = $3.71 <-- Price after tax
