First thing you do, you replace every letter by it's value.
5c - d = 5*3 - 7
Then, you calculate. Always, the Priority is for Multiplication and division, then Subtraction and Addition.
5*3 - 7 = 15 - 7
Then, you subtract
15 - 7 = 8
So, the answer is 8
Hope this Helps :)
