If the value of the argument is one third, then the values of the function f (x) = 3x + 5 = 6 is this correct?

First of all, we note that in order to determine what value the function will take (that is, f (x) or y) for a given value of the argument (that is, x), it is necessary to substitute instead of x the value specified in the setting condition, which takes x , and then calculate what value the function takes.
Hence, for x = 1/3 the function
f (x) = 3 * x + 5 takes the following value:
f (x) = 3 * x + 5 = [x = 1/3] = 3 * 1/3 + 5 = 3/3 + 5 = 1 + 5 = 6.
Thus, the answer f (1/3) = 6 is correct.