✓ Mathematica : cpu = 26.4034 (sec), leaf count = 18407
DSolve[-(a^2*(1 + Derivative[1][y][x])^2) + (-x + y[x])^2*(1 + Derivative[1][y][x]^2) == 0,y[x],x]
✓ Maple : cpu = 0.277 (sec), leaf count = 130
dsolve((y(x)-x)^2*(diff(y(x),x)^2+1)-a^2*(diff(y(x),x)+1)^2 = 0,y(x))