======================================================
Explanation:
3/7 = 0.42857 approximately
Pick a number between that value and 5, not including either endpoint. Let's say we pick x = 2
Plug x = 2 into the f(x) function
f(x) = (x - 5)(2x + 7)(7x-3)
f(2) = (2 - 5)(2*2 + 7)(7*2-3)
f(2) = (2 - 5)(4 + 7)(14-3)
f(2) = (-3)(11)(11)
f(2) = -363
The actual result doesn't matter. All we're after is whether the result is positive or negative. We see the result is negative. This means f(x) is negative when 3/7 < x < 5. The f(x) curve is below the x axis on this interval.
