Please help me and solve on paper

Problems 1. Verify that x1 (t) = cost is a solution of the ODE a" +tan(t)r' +sec (t)x = 0 -= 0). 3. Consider the following system of first-order constant coefficient homogeneous linear ODEs for x(t) and y(t): ( 2x' + y' - 2y = O y' + 4x + 2y = ( (a) Rewrite this system using the D-notation. (b) Eliminate x and find a differential equation satisfied by y(t). (c) Determine a general solution of this system