This looks like an assignment you have been set to develop your ability. If
we tell you the answer you will learn nothing.

You know that the If statement has three parts

The first part is the condition. In this case the condition is that total
sales are greater than a certain figure.

The next part is what you want if the condition is true i.e. the formula  for
the commission on the higher sales amount.

The third part is what you want if the condition is not true i.e. the formula
for the commission on the lower sales amount.

The format is

If(first part, second part, third part)

Now, You have a go at it and see if it works, then come back to the group with
what you have done if you have any problems. We can then explain the parts
you don't understand.