Answer:

27/28

Step-by-step explanation:

-2 3/4 + 3 5/7 = 27/28

Credits...

https://www.calculator.net/fraction-calculator.html?c5num1=-2+3%2F4&c5op=%2B&c5num2=3+5%2F7&ctype=5&x=111&y=18#mixednumbers