So 2 and 5/8 equals 21/8. Whenever you turn a mixed number into an improper fraction, multiply the whole number by the denominator and add the numerator. Since both fractions already have a common denominator, all you need to do is subtract the numerators and don't touch the denominator.
21/8 - 6/8 = 15/8 = 1 7/8
This cannot be simplified any further so I would write it as a mixed fraction.
Hope this helps!