Attend to the innermost parentheses that is farthest to the left,
so replace (1+5) by 6 and copy everything else over:



Attend to the innermost parentheses that is farthest to the left,
which is , so divide the 6 by the 1, getting 6 and 
copy everything else over:



Attend to the innermost parentheses that is farthest to the left,
which is , so subtract and replace it by 2 and 
copy everything else over:



Attend to the innermost parentheses that is farthest to the left,
which is , so subtract and replace it by (-3) and 
copy everything else over:



Attend to the innermost parentheses that is farthest to the left,
which is , so multiply and replace it by (-12) and 
copy everything else over:



Attend to the innermost parentheses that is farthest to the left,
which is , so replace it by 4 and 
copy everything else over:



And the final answer is



Edwin