You own a beauty salon where you not only cut hair but you also sell hair products. you buy a dozen barrettes for $18.00 and then sell them in your store for $2.00. you want to calculate your percent mark up.
to do so, you multiply 2 by 12, subtract 18 and then divide by 12. is this correct?

Yes your method is correct. But you will need to express it in percentage in the end.
So,
2 x 12 ( dozen ) = 24
24 - 18 = 6
6 / 12 = 0.5
$6 is how much you earned by selling 12 hair products.
Hence, $0.50 is how much you earned per bottle.
Transfering 0.5 into percentage would be 0.5 x 100.

Your percentage mark up would be 50 %

d = √[(x1-x2)2 + (y1-y2)2] = √[(-5-1)2 + (0-(-5))2] = √[(-6)2 + (5)2] = √(36+25) = √61 ≅ 7.8