A possibly better solution to this problem, given that it may not always be multiple choice, is to actually find the zeroes of this function.
First you would let f(x)=8x^2+4x+7=0
Now, since you were asked for an approximate answer, and this is a quadratic equation, you can use the quadratic formula, which says that the roots (the zeroes) of a function of this type (ax^2+bx+c=0) are equal to:
x=(b+sqrt(b^24ac))/2a
and
x=(bsqrt(b^24ac))/2a
Plugging in a=8, b=4 and c=7 simplifies to
x=(1+sqrt(15))/4
and
x=(1sqrt(15))/4
Plugging these into our calculators gives the last solution set.
You may also be familiar with factoring quadratic equations into the form (xn)(xm)=0 in order to solve them. With this one, a hint that you should use the quadratic equation is that the constant term (c or 7 in this problem) is prime, that is there are
no integers that you can multiply together to get seven
Jun 29

Michael P.