However, I'll make some assumptions:
1) you are holding a penny (a penny is 19 mm diameter which is almost 3/4 inch) at arms length;
your arm is 3 feet in length.
Converting all to feet:
0.75" is 0.75/12 feet or 0.0625 feet.
3 feet is to 0.0625 feet as some unknown distance x is to 84 inches (84/12 is 7 feet)
= 336 feet.
If the assumptions are not correct, then the results probably aren't either.