Question 12501
- 1/2(6r + 4s)  + 7(1/21s - 1/14r)
(-6/12r - 4/2s) + (7/21s - 7/14r)  multiply the nos.
(-1/2r  - 1/3s) +  (1/3s - 1/2r)  simplify the answer
 -1/2r  - 1/3s  +   1/3s - 1/2r   add all the nos. 
 = -2/2r = -r.
Since 2/2 is equal to 1, there's no need to put 1, so the answer is -r.