How can i solve this problem 6 2/3+5/4

Use PEMDAS
1. Parenthesis 
2. Exponents
3. Multiplication
4. Division 
5. Addition 
6. Subtraction 

So, youll want to make 6 2/3 into an improper fractions which is the 3 * 6 +2 /3 =20/3 +5/4 , now youll want common denominators which in this case it is 12,what u do to the bottom u do to the top, so 20/3 * 4/4 = 60/12 and 5/4 *3/3 = 15/12 SOOO
60/12 + 15/12 = 75/12 which simplified is 25/4 when you divide by 3. that equals out to be 6 1/4 :)

The answer is 9.7 cm