Find the potential φ(x,y)\varphi(x, y)φ(x,y) of an electrostatic field E=\mathbf{E}=E= =a(yi+xj),=a(y \mathbf{i}+x \mathbf{j}),=a(yi+xj), where aaa is a constant, i\mathbf{i}i and j\mathbf{j}j are the unit vectors of the xxx and yyy axes.