Given the triangle shown here, the first steps in the proof of the law of sines are:
a. set sina = b/k and sinb = k/a and solve for k in both equations.
b. set sina = k/b and sinb = a/k and solve for k in both equations.
c. set sina = -k/b and sinb = -k/a and solve for k in both equations.
d. set sina = k/b and sinb = k/a and solve for k in both equation.

Based on the given figure above, if you look at the right triangles, for angle A, b would be the hypotenuse and for angle B, a would be the hypotenuse in both triangles, k would be the opposite side. Base on this, we can conclude that the correct answer would be the last option or option D. Hope this answer helps. 

90 degrees and 90 degrees. Two right angles added to each other equals 180 degrees