Answer :

Make them both improper fractions.
77/10 - 19/5

Give them common denominators (multiply the second fraction by 2)
77/10 - 38/10

Subtract 
39/10 = 3 9/10
Question :
How to you solve 7 7/10- 3 4/5
Answer :
Convert to improper fractions. 
[tex] \frac{77}{10} - \frac{19}{5} [/tex]

Equate the denominator :
[tex] \frac{77}{10}- \frac{19}{5} = \frac{77}{10} - \frac{38}{10} [/tex]

subtract both fractions :
[tex]=\frac{39}{10}[/tex]

or equal :
[tex]3 \frac{9}{10} [/tex]