Draw a triangle; Then, divide the triangle into two right triangles. That line is h (stands for height).
We often label the angles in capital letters such as: A, B, and C.
The sides that is opposite of the angle is often labeled with the same letter but lower cased, such as: a, b, c.
sin© = h/a => (a)*sin© = h;
sin(A) = h/c => ©*sin(A) = h;
So, (a)*sin© = ©*sin(A); Or better yet, a/sin(A) = c/sin©.
We can also do a/sin(A) = b/sin(B).
Bring this together we get:
a/sin(A) = b/sin(B) = c/sin(C)
And
sin(A)/a = sin(B)/b = sin(C)/c