Question 260718
First solve in the parentheses, then move outwards. 
{{{2/3/3/4=2/(3*3*4)=1/(3*3*2)=1/18}}}
{{{(4/5)*(5/6)=2/3}}}
{{{1/2+(2/3/3/4)-(4/5*5/6)=1/2+1/18-2/3}}}
For fractions, you need a common denominator, use 18.
{{{1/2=(1/2)(9/9)=9/18}}}
{{{2/3=(2/3)(6/6)=12/18}}}
{{{1/2+(2/3/3/4)-(4/5*5/6)=1/2+1/18-2/3}}}
{{{1/2+(2/3/3/4)-(4/5*5/6)=9/18+1/18-12/18}}}
{{{1/2+(2/3/3/4)-(4/5*5/6)=(9+1-12)/18}}}
{{{1/2+(2/3/3/4)-(4/5*5/6)=(-2)/18}}}
{{{1/2+(2/3/3/4)-(4/5*5/6)=-1/9}}}