The equation
x = y²
is not a function because both (1,1) and (1,-1) satisfy the
equation, and a function by definition cannot have two
different y-values for the same x value.
To make a graph of y² = x, make a table of values:
x | y
9 | 3
9 |-3
4 | 2
4 |-2
1 | 1
1 |-1
0 | 0