What is the length of JK, to the nearest tenth of a millimeter?

5 = side a
3 = side c
angle B = 62 degrees
JK is 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 = (5^2 + 3^2) − ( 2 x 5 x 3) cos(62°) 
b^2 = (25 + 9) − 30 x 0.469 
b^2 = 34 - 30 x 0.469 
b^2 = 34 - 14.084 
b^2 = 19.916 
b = √19.916 
b = 4.463
Rounded to the nearest tenth is 4.5 mm.

Hope this helps!