Wolfram Alpha: How to define constants in a system of equations?

Solve [equations separated by commas] for [variables separated by commas]

In Wolfram Mathematica it's Solve[{eq1,eq2,...},{x1,x2,...}] so it should work in WA too.

The following works for me

solve  a+b+x -y = 5 , b-x-y = 10 for x and y 

or

solve  a+b+x-y = 5 , b-x-y +z= 10, x+z=0 for x ,y, z 

See the results here or here.