The expression is 

Place value chart for 18.9 and 6.72.

Line up the decimal points and draw a line below the numbers.

 First find the difference of the hundredths digit.

The difference of the hundredths digit, 0 is lesser than 2 so borrow 1 from tents digit. .

 First find the difference of the tenths digit.

The difference of the tenths digit, .

 First find the difference of the ones digit.

The difference of the ones digit, .

Next in tens digit only 1 is left. \ \

The difference of is 12.18.