Equations like this may be easier to understand in a context.
5 = 51 - a would fit this situation: You come home from the fabric store and discover that you only have 5 dollars left in your wallet. You remember that when you started the day you had 51 dollars in your wallet. So how much did you spend at the fabric store? You have lost the receipt and can't look there!
5 (dollars left in wallet) = 51 (dollars to start out) -a (amount spent at the fabric store)
The amount you spent must be equal to the difference between 51 and 5, so a = 51 - 5 = 46.
Wow! $46 worth of fabric! Happy Quilting!