For question 2, you take the smaller number, $\displaystyle -8$ from the biggest number $\displaystyle 24$, remembering when you take off a negative number you end up adding, i.e.

$\displaystyle 24-(-8)=24+8=32$

I'm not sure I understand how you've written question 3... did you mean $\displaystyle \frac{3}{4}-\frac{2}{4}+\frac{1}{2}+\frac{1}{3}+2+3$?

For question 5, you should write $\displaystyle 7\frac{3}{4}$ pounds as $\displaystyle 7.25$ (since $\displaystyle \frac{3}{4}=0.75$) and $\displaystyle 6\frac{1}{2}$ as $\displaystyle 6.5$ (since $\displaystyle \frac{1}{2}=0.5$). Then...

$\displaystyle 7.75+17.85+6.5=32.1$

Hope this helps