Factor the trinomial

To factor the trinomial y^2 + 4y - 21, we need to find two numbers that multiply to -21 and add up to 4. The numbers that fulfill these conditions are 7 and -3.

Therefore, we can rewrite the trinomial as:

y^2 + 7y - 3y - 21

Now, we can factor by grouping:

y(y + 7) - 3(y + 7)

Next, we factor out the common factor of (y + 7):

(y - 3)(y + 7)

Therefore, the factored form of the trinomial y^2 + 4y - 21 is (y - 3)(y + 7).