factor completely y^2 +yt-12t^2
$$\\y^2 +yt-12t^2\\\\=y^2+4yt-3yt-12t^2\\\\=y(y+4t)-3t(y+4t)\\\\=(y-3t)(y+4t)$$
Edited: Thanks Heureka
$$\small{\text{$ \mathbf{y^2 +yt-12t^2 = (y-3t)(y+4t)}$}}$$