Since the largest size of one of the dimensions is 10.5, it has to correspond with the long side of the card, the 5 in. side. To find the proportion of the sides, divide 10.5 by 5. Then, multiply the other side of the card by the quotient.
10.5/5 = 2.1
2.1 * 2 = 4.2
4.2in is the shorter side. Your question asks to round to the nearest tenths, the smallest place value of the answer is already the tenths place.