Solve System of differential equations

[parisse]

Correct syntax for second member is
s:=desolve(y'=[[1,0],[0,2]]*y + [3,4] ,t,y)
The reason is that y is a line vector, not a column vector. Indeed desolve returns a list of generic solutions, even for linear ODE, because you might get several generic solutions for non linear ODE. That mean that the solution y is s[0], i.e. a line vector.