Answer:
cos(B) = a/c
Step-by-step explanation:
The mnemonic SOH CAH TOA reminds you that ...
Cos = Adjacent/Hypotenuse
The leg adjacent to angle B is "a". The hypotenuse is "c", so the desired cosine is ...
cos(B) = adjacent/hypotenuse = a/c
N + d = 21
0.5n + 0.10d = 1.35
5n + 10d = 135
d = 21-n
5n + 210 - 10n = 135
-5n = -75
n = 15
d = 6