This is a polynomial with 2 terms. The minus sign in front of the 9 indicates subtraction. Everything before that minus sign is the first term and everything after that minus sign is the second term.

Each term consists of a series of things being multiplied together. When no other operation is clearly given, assume that multiplication is the operation. Let us consider the first term.

s^2(t - u) means s to the second power times some quantity in parentheses. In this case, the thing in parentheses is t - u.

Let us consider the second term.

9t^2(t - u) means that we multiply 9 times t to the second power times some quantity in parentheses. Again, the thing in parentheses is t - u

Both terms involve multiplying 2 or 3 different things together. Which of those things shows up in both terms?

s^2 only shows up in the first term so that won't be a common factor between both terms.

Likewise, 9 and t^2 only show up in the second term. However, (t-u) shows up in both terms and can be factored out.

(t-u)(s^2 - 9t^2) is the factored form.

Another way of approaching this problem is to make a new rule and do substitution. For example, I am going to decide that I don't want to write (t-u) so much, so I set up this rule: A = (t-u). Now, with this rule I can replace (t-u) with A every time it appears in the problem:

s^2A - 9 t^2 A

From this we can see that A appears in both terms and can be factored out:

A(s^2 - 9 t^2)

Finally, we go back to our rule and replace the A with what it really represents:

(t - u) (s^2 - 9 t^2) which is the same answer.

To check your work, use the distributive property to multiply your answer out an see if you get the original problem.

I hope that helps.

Good luck,

Robert