Since: Bx/a -ay/b+a+b =0.
So a and b do not equal to zero. 
Then make the two sides of the first equation multiply a.
That is:Bx-a^2y/b+a^2+ba=0. 
Use this equation to minus the first equation.
That is:(a^2/b-a)y+ab-a^2=0. 
Now you can get the value of y.
It should equals b. 
Put y=b into the first equation
and you can know the value of x which should be
 x= -ab/B.