If you have $45 and they take 2.1% of it how much do you have left ? if you have $765 and they take 2.1% of it how much do you have left ? if you have $1300 and they take 2.1% of it how much do you have left ?

First you need to make a multiplier for 100-2.1%, to do this you minus the values and divide it by 100:
100-2.1 = 97.9
97.9/100 = 0.979

Then you multiply all the questions by it:
$45 * 0.979 = $44.055
$765 * 0.979 = $748.935
$1300 * 0.979 = $1272.7

Hope this helps! :)