Explanation:
Not sure what ' tan x' is doing in there, because it is not true
Anyway:
Multiply both sides of the equation by k- y to get
y = (k-y) sin2 / cos2 now multiply both sides by cos^2
y cos^2 = (k-y) sin^2 expand R side
y cos^2 = ( k sin^2 - y sin^2 ) add y sin^2 to both sides
y cos^2 + y sin^2 = k sin^2 factor out 'y'
y ( cos^2 + sin^2) = k sin^2 remember : sin^2 + cos^2 =1
y ( 1) = k sin^2
y = k sin^2