Step-by-step explanation:
The answer is:
∠
B
=
sin
−
1
(
20
⋅
sin
(
30
∘
)
15
)
You have to use the law of sines, just as you thought. This law is:
a
sin
∠
A
=
b
sin
∠
B
=
c
sin
∠
C
We only need the first equation here, let's fill in the numbers.
15
sin
(
30
∘
)
=
20
sin
∠
B
We use cross-multiplication, then we find:
sin
∠
B
=
20
⋅
sin
(
30
∘
)
15
then by taking the inverse sine, we find
∠
B
=
sin
−
1
(
20
⋅
sin
(
30
∘
)
15
)
