A>0 Assuming a function

f(x)=Sum(0 to infinity) c_{n}x^{n}

is the solution to the differential equation:

f''(x)=af(x)

with the conditions f(0)=1 f'(0)=0 find a formula for the coefficients of f. what is its radius of convergence?

completely lost and do not understand the problem someone point me in the right direction??