4/b=1/a+3/c=(c+3a)/ac
4/b^2=3/ac,
4/b^2=(c+3a)^2/4(ac)^2=3/ac
(c+3a)^2=12ac
(c-3a)^2=0
c=3a
b=2a,
ax^2+bx-c=0
化成
ax^2+2ax-3ax=0
a(x-1)(x+3)=0
x1=1,x2=-3