Add like terms with the same sign by adding the numerical coefficients and keeping that sign:
ab^2 + 2ab^2 = 3ab^2
-2xy-3xy = -5xy
-4a+(-6a) = -10a
Add like terms with opposite signs by subtracting the smaller term from the larger term. Keep the sign of the larger term:
-5a+a = -4a
7y+(-3y) = 4y
Subtract a negative term by following the formal rule -(-a) = a:
2x-(-3x) = 2x + 3x = 5x
3-(-6) = 3+6 = 9
-6y -(-3y) = -6y +3y = -3y
Combine a long series of terms by grouping like terms together for ease of calculation:
6a -7b-3a+4b = 6a-3a-7b+4b = 3a-3b
Remember that a negative term times a negative term yields a positive term. A negative term times a positive term yields a negative term.
Multiply (and divide) like terms with an odd number of negative signs by multiplying the numerical coefficient and and putting in a negative sign. So, the result is negative:
-7y*-3y*-2y = -42y.
With an even number of negative signs the result is positive:
-7y*-3y = 21y