How do you simplify (7−3)(2−27)?

Step 1
If you ignore the rule of imaginary numbers which states that
${\displaystyle \sqrt{-1}=i}$ and ${\displaystyle i^2=-1}$
you would be tempted to multiply all terms together to get
${\displaystyle 14\sqrt{81}=14\times 9=126}$
which would be incorrect.
To keep the validity of the complex number system, always deal with the imaginary part first. To do this, rewrite the equation:
${\displaystyle (7\sqrt{3}\times \sqrt{-1})(2\sqrt{27}\times \sqrt{-1})=(\sqrt{-1}\times \sqrt{-1})(7\sqrt{3}\times 2\sqrt{27})}$
Now you can multiply terms within the parentheses.
$(\sqrt{-1}\times \sqrt{-1})=i\times i=i^2=-1$
and ${\displaystyle (7\sqrt{3}\times 2\sqrt{27})=14\sqrt{81}=14\times 9=126}$
Multiply these together and you get ${\displaystyle 126\times -1=-126}$