Consider the following four differential equations:

EQ 1:  \(\frac{{{\rm{dy}}}}{{{\rm{dx}}}} - {\rm{y}} = {\rm{\pi }}\)

EQ 2: \({\rm{\;}}{\left( {\frac{{{\rm{dy}}}}{{{\rm{dx}}}}} \right)^2} + 3{\rm{y}} = 36\)

EQ 3:  \(\frac{{{{\rm{d}}^2}{\rm{y}}}}{{{\rm{d}}{{\rm{x}}^2}}} + 5\frac{{{\rm{dy}}}}{{{\rm{dx}}}} + 6{\rm{y}} = {\rm{\;\;}}0\)

EQ 4: \({\rm{\;}}\frac{{{\rm{dy}}}}{{{\rm{dx}}}} + {\rm{y}} = {\rm{tany}}\)

Among these four differential equations which is/are linear differential equation(s)?