Answer:
[tex]\text{Third Option}[/tex]
Step-by-step explanation:
[tex]\text{Solution:}\\f(x)=64x^2+6=31\\\text{or, }64x^2=31-6=25\\\\\text{or, }x^2=\dfrac{25}{64}\\\\\text{or, }x=\pm\sqrt{\dfrac{25}{64}}\\\\\text{or, }x=\pm\dfrac{5}{8}[/tex]
