C4 binomial q

I'm stuck on part (c), I have tried setting root5 equal to 1/(1+2x)^-1/2 but that gives me -0.1, which is what is given, but I don't know what to sub that into because when I sub it into the expansion for 1/(1+2x)^-1/2 I don't get 2.23

If you sub $x=-0.1$ into your expression you'll get $\dfrac{1}{\sqrt{0.8}}$

$=\sqrt{1.25}= \sqrt{\frac{5}{4}}=\frac{\sqrt{5}}{2}$

So, whatever your expansion works out to, you'll need to double it to get the approximation of $\sqrt{5}$