Error message in this code

HI there,

I m getting an error message . Could someone help me

v1 := int(cos(tau)*g(tau), tau = t0 .. t);
int(cos(tau) g(tau), tau = t0 .. t)
v2 := int(-sin(tau)*g(tau), tau = t0 .. t);
int(-sin(tau) g(tau), tau = t0 .. t)
soln := C1*y1+C2*y2+v1*y1+v2*y2;
C1 y1 + C2 y2 + (int(cos(tau) g(tau), tau = t0 .. t)) y1

+ (int(-sin(tau) g(tau), tau = t0 .. t)) y2
soln := combine(soln);
(int(-y2 sin(tau) g(tau) + y1 cos(tau) g(tau), tau = t0 .. t))

+ C1 y1 + C2 y2
eval(soln, t = t0) = 0, eval(diff(soln, t), t = t0);
C1 y1 + C2 y2 = 0, -y2 sin(t0) g(t0) + y1 cos(t0) g(t0)
solve({%}, {C1, C2});
soln := eval(soln, %);
Error, invalid input: eval received (C1*y1+C2*y2 = 0, -y2*sin(t0)*g(t0)+y1*cos(t0)*g(t0)), which is not valid for its 2nd argument, eqns