x - 3
Factors of the polynomial can be solved to get the zeros of the function (where the graph crosses the x-intercept). If we examine this graph we see that it crosses the x-int at 3, so 3 is a zero of the function. This would be (x-3) as a factor because when solved you get x = 3 (see below).
0 = x - 3
+3 +3
3 = x