Start with the given expression
In order to subtract these two fractions, these fractions need to have a common denominator.
In order to do that, we simply find that the LCM of 8 and 4 is 8 (note: if you need help with finding the LCM, check out this solver)
Now we need to get each denominator to 8
Multiply by 
Multiply and to get 
Since both fractions have a common denominator of 8, we can now combine the fractions
Combine the fractions
Subtract the numerators
So

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