Answer:
the 4" side of the card was scaled up to 10.5
then the scaling factor is 10.5/4 = 2.625.
The 5 side would be the length 5 x 2.625 = 13.125 which exceeds the maximum dimension of 11.
Thus, it is the 5 side which is scaled up to 10.5 and the scaling factor is therefore 10.5/5 = 2.1.
The scaled up length of the 4 side is thus 4 x 2.1 = 8.4.