When asked to factor the x^2-121 a student gives the answer (x-11) (x-11) what is wron with this answer?

one minus sign should be a plus sign

Given expression is x² - 121

This is called Difference of Squared terms, we have a formula that is given as : -

a² - b² = (a - b) · (a + b)

Now using the above formula in the given expression, we get : -

x² - 121

x² - (11) ²

here a = x and b = 11

x² - (11) ² = (x - 11) · (x + 11)

but it says that student gave the answer as (x - 11) · (x - 11).

So, student's answer should be (x - 11) · (x + 11) instead of product of two (x - 11) terms.