Please reread what you posted.
Does that maks any sense to you?
Your labels are confusing . . . I'll revise them.
I will assume that we know: . .Given a triangle with opposite sides respectively,
show that the area of is given by: .
. . (the area is one-half the product of two sides and the sine of the included angle)
Multiply  by .
From the Law of Sines: .
Substitute into : . .
Since , then: .
. . and: .
Substitute into : . ta-DAA!