If the general solution of a differential equation is y² + 2cy - cx + c² = 0, where c is an arbitrary constant, then what is the order of the differential equation?

What is the degree of the following differential equation?

$\rm x=\sqrt{1+\frac{d^2y}{dx^2}}$

Which one of the following differential equations has the general solution y = ae^x + be^-x?

What is the solution of the following differential equation?

$\rm \ln\left(\frac{dy}{dx}\right)+y = x$