First we find how much was left as 1/8 full
16.8 times 0.125
2.1
Now add this to the amount that was added to reach full gauge
2.1 +16.8
18.9
Next we find out what 3/4 of a full tank is
18.9 times 0.75
14.175
So, the gas tank has 14.175 gallons when it is 3/4 full