The expression is 

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 = .

Next find the difference of the tenths digit.

Notice that it is not possible to subtract 2 tenths from 7 tenths,

so 1 unit must be borrowed from ones column.

The difference of the tenths digit =.

Next find the difference of the ones digit.

The difference of the ones digit = .

The difference is .