Answer:
[tex]y = x^{2} + 3x + 1[/tex]
Step-by-step explanation:
Perimeter equals the sum of the lengths of the sides.
Let y = the length of the fourth side
[tex]4x^{2} + 3x - 4 = x^{2} + 2x + 15 + x^{2} -x - 10 + x^{2} - x - 10 + y[/tex]
[tex]4x^{2} + 3x - 4 = 3x^{2} - 5 + y[/tex]
[tex]x^{2} + 3x + 1 = y[/tex]