Factor by grouping

3r^{2} + 8r -3

Where

a = 3

b = 8

c = -3

You use the product of a*c = -9

Now ask yourself what factors of -9 could add up to 8?

it's gonna be 9 and -1 so you write that out in place of 8r

3r^{2} + 9r - 1r -3

Now separate your common terms

3r**(r + 3)** - 1**(r+3)**

**And finally
**

**(3r - 1)(r + 3)
**