In other words, can 9y - 3x - 4 = 0 be solved for y, and, if so, is there always exactly one y value for every input (x) value?
9y = 3x + 4
Thus, y = (3/9)x + 4, or y = (1/3)x + 4
No matter which real x you choose, y is always assigned exactly one value by this rule. So, yes, this is a function.