A term of an expression is the different pieces that are separated by the plus and minus signs.
A coefficient of a term is the number part of a term that is attached to the variable.
1) 7x + 8y
The terms are 7x and 8y. The coefficient of the first term, 7x, is 7. The coefficient of the second term, 8y, is 8.
2) a - b
The terms are a and b. The coefficients of both of these terms are 1, however they are not written because they are implied.
3) 3m^2 - 6n
The terms are 3m^2 and 6n. The coefficient of the first term, 3m^2, is 3. The coefficient of the second term, 6n, is 6.