somebody help me with this please .

Step-by-step explanation:
12. Cos 60° = 8/c
0,5 = 8/c
0,5 c = 8
c = 16
D² = V16²-8²
= V256-64
=V192 = V16×12 = 4V12
= 4V4×3 = 8V3
13. Cos 30° = 6/b
V3/2 = 6/b
V3 b = 12
b = 12/V3
b/Sin B = a /sin A
b/Sin90° = 6/ sin 60°
b = 6
1 V3/2
b× V3 = 6
2
b = 6× 2/V3
= 12/V3