What is the length of AB¯¯¯¯¯?Enter your answer as a decimal in the box. Round only your final answer to the nearest tenth.

If BC is your shortest like, that would make it line A of the pythagorean theorem. AC is line B, and that would make line AB the hypotenuse, or line C.
So....
a^2+b^2=c^2 
12^2+18^2=c^2
12*12=144
18*18=324
324+144=c^2
468=c^2
Because it is squared, you can divide by 2 and get your answer to line AB.