To answer this question you would have two steps involved. The first step would be to find the percentage of $42 that gives us $2.94. Set up an equation 42x=2.94. Divide both sides by 42 to get the decimal 0.07. This equal 7%. Take this percentage multiplied by the $58. You need to change the 7% into 0.07 to be able to calculate it. 0.07 x 58 = $4.06 sales tax.