Put 4 where "a" is and do the arithmetic.
[tex]12-\dfrac{a}{4}=12-\dfrac{4}{4}=12-1=11[/tex]
The value of the expression is 11.
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This is very different from
[tex]\dfrac{12-a}{4}=\dfrac{12-4}{4}=\dfrac{8}{4}=2[/tex]
In this latter case, parentheses are required when the expression is rendered using plain text: (12 -a)/4.
