Which two integers is -√21 between?

Which is an equation for the nth terms of the sequence 12,15,18,21

At the many chips cookie company they are serious about the number of chocolate chips in their cookies they claim that each cookie hasn't c chips. if their claim is true there will be 200 chips in 10 cookies

Δabc is reflected across line l to form δ alblcl, and a¯¯¯al¯¯¯¯ intersects line l at point d. which equation is not necessarily true?