the only criteria, really, to be a function is to assign exactly 1 y-value to every valid x-value. not 2 or 0 y-values.
we see here, x = 2 has one assigned y-value : y = 3.
x = 4 has y = 9.
x = 6 has y = 10.
x = 9 has y = 5.
there is no indication that there are more valid x-values.
each x-value used has exactly 1 y-value.
so, the criteria is fulfilled. it is a function.