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!