checking zeros
1.x⁴-2x²+5x+6 =0
x=1, 1-2+5+6= 0, 10=0 false , not first one
2. x³-2x²-5x+6 =0 correct answer
x=1 , 1-2-5+6=0, 0=0 true
3. x³-2x²+5x+6 =0
x=1, 1³-2*1²+5*1+6 =0, 1-2+5+6=0, 10=0 false , not third one
4. -x⁴-2x²+5x+6 =0
x=1, -1-2+5+6 =0, 8=0 false, not this one
