'm stuck! I'm trying to find x using the cosine rule from:

The answer is; x = 7.23 from:

How?

In another question i have to find the value of x which has the minimum value:

$\displaystyle b^2 = x^2 + x + 7$

I'm guessing i have to complete the square? but $\displaystyle b = b^2$ so do i take the route of the whole thing?