dsolve(Eq(Derivative(p(t), t), -(u3 + u4) * p(t) + exp(-t * (u1 + u2))),
    hint='1st_linear_Integral')
dsolve(Eq(Derivative(p(t), t), -(u3 + u4) * p(t) + exp(-t * (u1 + u2))).
    subs(u1 + u2, u5), hint='1st_linear_Integral')
SymPy - Problems when using to many parameters in dsolve