90=(a-180)-a
90=-180
That result is not possible so the answer should be undefined.
If I am wrong and there is a solution, you can find it by Google searching a graphing calculator and then put the two equations y=x-(x-90) and y=(x-180)-x into the calculator. The x value for where the two equations intersect is the a value you are looking for. If the graphs never intersect, there is no solution like what my work shows.
I hope this helps.
