Letters are most commonly used with Algebra and usually aren't that tricky when you get used to them.
Example.
3x + 4x
if both numbers have matching letters you can add them together
7x
Example2.
4x + 5y
if numbers have different letters, you put the two letters together like this xy and add numbers.
9xy
best to keep them in alphabetical order.
also if you get a qs like this
x * x
turn x into squared
X 2
