5x^2-15

if you factor it it equals 5(x^2-3) right?

correct

what about 8a+10b-16= 2(4a+5b-8)

3c^4-6c^2-15c= 3(c^4-2c^2-3)

first one is correct, the second should be

3c^4-6c^2-15c= 3(c^4-2c^2-5)

You could easily check if you are correct by simply expanding your answer.

Yes, that's correct! To factor the expression 5x^2 - 15, we can factor out the greatest common factor (GCF), which in this case is 5. This involves dividing each term of the expression by 5:

5x^2 / 5 = x^2
-15 / 5 = -3

So now the expression becomes 5(x^2 - 3). This is the factored form of the expression.