hoa can i solve this system ?

How can i solve this problem:

[A]{x}+[B]{x'}+[C]{x''}={K}

A,B,C:known matrices K:known vector (nx1)

and x'' derivative of x' and x' derivative of x(unknown)

[Charis Theodorou]

Also can you remind me how i write the ode45 when i have a vector(nx1) and not a number?

 I call this function with ode(@diplomatiki, [0 20], zeros(n+3,2))

It might be an error regarding dimensions since I am not sure how to cope with vectors