Answer:
(6, 8)
Explanation:
The rectangle ABCD has vertices at A(0,0), B(0,4), C(3,4) and D(3,0). The length of the sides is calculated using the distance formula:

Therefore:

If the length of each side is doubled and point A stays the same. Let us assume that the new point of C is C'(x, y). Therefore C would be the midpoint of segment |AC'|:

Therefore C'=(6,8)
The new coordinate of point C would be (6, 8)