Question

I have a few problems on combining like terms please help:

Answer 1

Combing like terms is actually pretty simple. As long as you can do basic addition, multiplication, division, and subtraction, you should be fine. Like terms are two or more terms that have the same variable(s) -- sometimes like terms can be two or more regular numbers without variables as well. When combing like terms without a variable, just perform the operation as normal (i.e. 2 + 5 = 7). When combing like terms with a variable, perform the operation on the coefficients (i.e. 10x + 5x = (10+5)x = 15x). 

1) 12r - 8 - 12
Here the like terms are -8 and 12.
-8 - 12 = -20
12r - 20

2) n - 10 + 9n - 3
Here the like terms are n and 9n and -10 and -3.
To make it easier you can reorder the expression to be n + 9n - 10 - 3.
n + 9n = (1 + 9)n = 10n
-10 - 3 = -13
10n - 13

3) -4x - 10x
Here the like terms are the only two terms in the expression, -4x and -10x.
(-4 - 10)x = -14x
-14x

4) -r - 10r
Here the like terms are the only two terms in the expression, -r and -10r.
(-1 - 10)r = -11
-11r

Answer 2

$12r-20\\ 10n-13\\ -14x\\ -11r$