clear variables
syms u_C(t) R C U_0 U_g
ode1 = diff(u_C,t) == -1/(R*C)*u_C + U_g/(R*C)
ode1(t) = 
cond1 = u_C(0) == U_0
cond1 = 
uCSol(t) = dsolve(ode1,cond1)
uCSol(t) = 
syms u_C(t) R C U_0
ode2 = diff(u_C,t) == -1/(R*C)*u_C + U_0/(R*C)
ode2(t) = 
cond2 = u_C(0) == -U_0
cond2 = 
uCSol(t) = dsolve(ode2,cond2)
uCSol(t) = 
syms x(t)
ode3 = diff(x,t) == -1000*x + 10000
ode3(t) = 
cond3 = x(0) == -10
cond3 = 
xSol(t) = dsolve(ode3,cond3)
xSol(t) = 
fplot(xSol(t), [0 0.01])
ylabel('x(t)')
xlabel('t')