Question

s=a+b+c/2 solve for b
the correct answer is b=2s-a-c I would like to know how this answer came up how do I find b

Answer 1

"s=a+b+c/2 solve for b
the correct answer is b=2s-a-c I would like to know how this answer came up how do I find b "

You cannot, this shows the importance of parentheses and the priority of operators (Parentheses first and foremost, then exponents, then multiplication and division, then addition and subtraction)

The given problem was probably printed like this:
s=
a+b+c
---------- ...(1)
2

in which case it should be transcribed as
s=(a+b+c)/s

[and not s=a+b+c/2 ...(2)]

(1) solves to be b=2s-a-c,
(2) solves to be b=s-a-(c/2)