I need to know how to factor by grouping and how to factor completely in Algebra 2
Factoring is a big topic - too big to address adequately in this short space. Try looking over this lesson on factoring.
If you have specific questions, or would like a specific equation factored, post it here and we'll help.
Group as follows:
(126x3-54x2) - (210x-90)
You can factor an 18x2 out of the terms in the first parentheses and a 30 out of the terms in the second parentheses:
18x2(7x-3) - 30(7x-3)
Now factor out the (7x-3):
Finally, I can factor out a 6 from the terms in the first parentheses:
Check by multiplying out the factored expression (FOIL) to verify that you get the original expression back.