How do I solve this quadratic equation

Question

the quadratic f(x)=ax^2+bx+c with f(-2)=0 and -b/2a=1 solve f(x)=0

Answer 1

f(-2)=0=4a-2b+c

b=-2a

4a+4a+c=0

c=-8a

quadratic formula gives

1±(1/2a) sqrt[4a² - 4a(-8a)]=1± (1/2a)*2sqrt(a²+8a²)=1±3, i.e. the roots are 4 and -2