Find a finite set of real functions that spans the solution space to the following homogeneous ordinary

Question:

Find a finite set of real functions that spans the solution space to the following homogeneous ordinary differential equations:
(a) uʹ - 2u = 0
(b) uʹʹ+ 4u = 0
(c) uʹʹ - 3uʹʹ= 0
(d) uʹʹ + 4uʹ+3u =0
(e) uʹʹ + uʹ + u = 0
(f) uʹʹʹ - 5uʹʹ = 0
(g) u(4) + u = 0