To factor,
1) First multiply coefficient of a² and constant no,
That is,
3×(-42)=-126
Since the resultant no is negative, you should find two such factors of 126 which will give us the coefficient of a (=11) on subracting those factors.
2) Find the factor
126=2×3×3×7
=18×7
18 and 17 are factors of 126
Also,18-7 =11.
So they are required factors for factoring,
Once you have understood above steps you can solve on your own. All you need to do is split 11 into factors ,take common terms and you will get answer.
Answer:
3a²+11a-42
=3a²+(18-7)a -42
=3a²+18a-7a-42
=3a(a+6) -7(a+6)
=(a+6)(3a-7)
