In order to estimate, start off by rounding both numbers to the nearest whole number. -7.94 can be rounded to -8 because the first number to the right of 7 is greater than 5. 2.31 can be rounded to 2 because the first number to the right of the 2 is less than 5.
After rounding, the expression becomes -8-2, which is equal to -10.
678 is what you have left but if you would put it as a fraction of what you have left now it would be 678/1012... which then take and put it as a decimal which is a form of a mixed number = .6699
