For 1:
You need to make a right angled triangle. This is quite trippy, but note that:
\sqrt{40}^{2} + 3^{2} = 7^{2}
For 2:
\frac{2}{3}\sqrt{45} = \sqrt{x} \Rightarrow \left ( \frac{2}{3}\sqrt{45} \right )^{2} = x
For 3:
Equation should be:
2n^{2} = 5 + 9n
Because it's twice a number...