Substitution with systems of partial differential equations (algebraic)

Regarding systems of partial differential equations, and the substitution function. Let's say I have the following equations:

(1) d y(t) / dt = x(t) + 2*y(t)

(2) d x(t) / dt = u + x(t) + y(t) (u is not dependent on time)

(3) V(t) = x(t) + y(t)

--> Now, let's say I want to take the derivative of equation 3 with respect to time, AND I want to plug in equations 1 and 2 into this time derivative of 3. How can I do this with Matlab?