Answer:
Whenever the denom is 0, that's where it's undefined -- or in your case "restrictions"
So factoring 2x^2 + x - 21 gets you (x-3)(2x+7). That means 3 and -7/2 are the restricted values.
To simplify, I'm going to factor it. (Assuming what you wrote is 8x^2 for the numerator)
[(4x-7)(2x+7)] / (x-3)(2x+7)
You can cancel 2x+7 out,
so you get 4x-7/x-3