The solution is x(t) = -t*cos(2t) + t*sin(2t) - cos(2t).
To solve the given initial-value problem, we can use the method of undetermined coefficients.
First, we find the complementary solution by solving the homogeneous equation (d²*x)/(d*t²) + 4x = 0.
The characteristic equation is r² + 4 = 0, which has complex roots ₁ = -2i and r₂ = 2i.
So, the complementary solution is X