First let's convert your statistics to metric.
5'8.0" ~ 1.73 cm 155 lbs ~ 70.5 kg
5'8.0" ~ 1.73 cm
155 lbs ~ 70.5 kg
Should be 1.73 m
BMI = (weight in pounds * 703 ) / (height in inches)².
155*703/68*68= 23.565