How do I calculate f(f(1)) = (x-1)(x-2) ?

f(f(1)) = (x-1)(x-2)

First....we want to find f(1),,,,this just says to "plug" 1 into the function and evaluate

So we have

(1-1)(1-2) = (0)(-1) = 0

Now.... f(f(1))  ... says we want to take this result and "plug"  it  back into the function, again...so we have

f(f(1)) = f(0) = (0-1)(0-2) = (-1)(-2) = 2