I'm not sure whether you mean this:



or this:



The left to right PEMDAS rule says the second is the correct way, because
the division by "2" must be done before we come to the "a", and so the
"a" will be multiplied, and it is not part of the division.  But I
kind of think you meant the first one because most students on here
usually don't follow the left to right PEMDAS rule.

To include the "a" in the denominator, you would have to put parentheses
around the "2a", like this

d=vf^2-vi^2/(2a)



Multiply through by "2a"



Get all terms containing "a" on the left side:

 

Factor out 2a on the left



Divide both sides by "2(d-vf²)"







That's an OK answer, but sometimes teachers don't like
any answer left beginning with a negative sign if there
is any other possible way to write it.  In case your
teacher is one of those, reverse the order of terms in
the parentheses in the denominator to make the negative 
term come first:



Now factor a (-1) put of the parentheses in the denominator 
(it'll change the sign of the +d to -d.



Now the negative sign on top cancels the "(-1)"
on the bottom and you have:



Edwin