Answer:
8x+8
Explanation:
perimeter = lengths of all sides added up
we want all sides to add up to 6x + 4
so far, we have 2x-7 and -4x + 3
side 1 + side 2 + side 3 = perimeter = 6x + 4
2x - 7 + (-4x + 3) + side 3 = 6x + 4
-2x -4 + side 3 = 6x + 4
add (2x+4) to both sides to isolate side 3
side 3 = 8x + 8