The answer is (7y + 6)^2 but idk how to get that
The first thing you should have noticed is that $\displaystyle 49y^2= (7y)^2$ and $\displaystyle 36= 6^2$. For any a and b, $\displaystyle (a+ b)^2= a^2+ 2ab+ b^2$. With a= 7y and b= 6, that would give [tex]49y6^2+ 2(7y)(6)+ 36= 49y^2+ 84y+ 36[tex], exactly what you have here.