$\displaystyle \sqrt[4]{2x + 1} - 0.1 < \frac{1}{2}x + 1 < \sqrt[4]{2x + 1} + 0.1$

I don't know how to do this because there is a 4th root in there which makes some terms have 4th powers. The answer is -0.368935 < x < 0.677669 but I don't know how to get to only 1 x in the middle.