K = F + 459.67
Subtract K on both sides:
0 = F - K + 459.67
Subtract F on both sides:
-F = -K + 459.67
Divide each term by -1 so that the variables are positive:
F = K - 459.67
You can download this app called Symbolab but the answer is -144xy^2
Here, it is solved.
you only have to know:
The sum of the sides of a quadrilateral is equal to 360º.