I do not know how to solve this problem... bad when I'm in 10th grade isn't it?
x + 3.5 + y = 21
Any real number is a solution to one of the two variables of the equation, there is no unique solution.
We can only give you the solution to one variable when the other is given, for example:
When x = 0
$\displaystyle 0 + 3.5 + y = 21$
y = 17.5
When x = 1
$\displaystyle 1 + 3.5 + y = 21$
y = 16.5
So, we cannot give you a fixed solution to this problem.