Perimeter of the triangle = Sum of three sides
= 8y^{2} – 9y + 4
Sum of two sides = 3y^{2} – 5y + 4y^{2 }+ 12
= 7y^{2} – 5y + 12
∴ (8y^{2 }– 9y + 4) – (7y^{2} – 5y + 12)
= 8y^{2} – 9y + 4 – 7y^{2} + 5y – 12
= y^{2} – 4y – 8
Hence third side = y^{2} – 4y - 8