Question

what is a function? if the input is 72 what is the output

Answer 1

a function is basically a rule that relates an input value to an output value. There can be only one output value for any input value.

So, the table
1 4
3 9
0 2
is a function, but
1 4
4 8
1 5
is not, since there's no way to determine which value corresponds to 1.

You have given us an input value, but there's no way to tell the output unless you provide a table as above, or a rule connecting input values to output values.

For example, if the rule is

f(x) = x-20
then
f(72) = 72-20 = 52

Answer 2

if the input is 24 what is the output?
the table is: the input table:
x,4,8,12,16
the output table is:
y,1,2,3,4

Answer 3

9