Find the general solution of the given second-order differential equation. y'' − y' − 20y = 0

Taking common factors

y (y'-y-20)=0

there are two possible solutions

y=0

or

y'-y-20y=0

thus using the formula

where a is 1 b is -1 and c is -20

but as you substitute the answer inside the root is negative and that cant be possible so the question has no zero and the only possible solution is

y=0