Answer:
Given: In ΔABC ,

To prove that:

[Given]
In ΔADB
The sine angle is defined in the context of a right triangle is the ratio of the length of the side that is opposite that angle to the length of the longest side of the triangle.
[By definition of sine] .....[1]
Multiplication Property of equality states that you multiply both sides of an equation by the same number.
Multiply by c to both sides of an equation [1] we get;

Simplify:
......[2]
Now, In ΔACD
Using definition of sine:

Multiply both sides of an equation by b;
[Multiplication Property of equality]
Simplify:
......[3]
Substitute [3] in [2];
......[4]
Division property of equality states that if you divide both sides of an equation by the same nonzero number the sides remains equal.
[4] ⇒

Therefore, the missing statement in step 6 is;