To give a valid answer, it is necessary to know how far from your eye the 0.75" measurement happens to be.

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.