Please help me with this differential equation question. I don't understand how to do it.

a) For which values of k does the function y=3cos(kt) satisfy the dierential equation;

25(d²y/dt²)= -4y?

b). For which values of r does the function y=e^rx satisfy the dierential equation;

(d²y/dx²) - 6(dy/dx) +8y = 0

Thank you in advanced!!