What is the length of ab? enter your answer in the box, round only your final answer to the nearest tenth.

Side c = 18 
Angle B = 79°
Side a = 12
AB = side b
Use the Law of Cosines to find the length of side b. 
b^2 = a^2 + c^2 − 2ac cos(B) 
b^2 = (122 + 182) − ( 2 x 12 x 18) cos(79°) 
b^2 = (144 + 324) − 432 x 0.191 
b^2 = 468 - 432 x 0.191 
b^2 = 468 - 82.429 
b^2 = 385.571 
b = √385.571 
b = 19.636

Rounded to the nearest tenth is 19.6 cm.

Hope this helps!