You can use fplot()
syms t v
v = dsolve('D2v=-2*Dv-3*v+4*10*sin(5*pi*t)','v(0)=0','Dv(0)=0','t')
fplot(v)
Also, defining ODEs as string is not recommended. You can define it using symbolic variables as follow
syms v(t)
dv = diff(v, 1);
d2v = diff(v, 2);
eq = d2v == -2*dv-3*v+4*10*sin(5*pi*t);
conds = [v(0) == 0; dv(0)==0];
v = dsolve(eq, conds, t)
fplot(v)
