9(a) work out the value of 5c – d when c = 3 and d = 7

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 :)

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