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The general solution is $u(x,y) = f(y - (2/5)x)$, where f is an arbitrary differentiable function of one variable. This form arises because u is constant along the characteristic curves $y = (2/5)x + c$, so u depends only on the invariant $c = y - (2/5)x$.

Conditions: u is constant along the characteristic curves.; f is an arbitrary function of one variable.

The general solution is $u(x,y) = f(y - (2/5)x)$, where f is an arbitrary differentiable function of one variable. This form arises because u is constant along the characteristic curves $y = (2/5)x + c$, so u depends only on the invariant $c = y - (2/5)x$.

Conditions: u is constant along the characteristic curves.; f is an arbitrary function of one variable.