Decide whether y is a function of x.
1. 5x - y = 10
2. x^2 + y^2 = 4
#1 is a line, so there is only one place where a vertical line cuts the graph. It is a function.
#2 is a circle. Many vertical lines intersect the circle in 2 points, so it is not a function.
Think about it. If any value of y satisfies the equation, so does -y, since y^2 is the same in both cases.