Answer:
S = 
Step-by-step explanation:

The answer is 12+3r because 4 times 3=12 then the 3r which you cannot add because they are no others variables that are r
(a/b)(c/d)=(ac/bd)
Therefore:
(5/4).(2/5)=(5*2)/(4*5)=10/20=1/2
(3/8).(-1/2)=(3*(-1)) /(8*2)=-3/16
(2/5)(5/2)=(2*5)/(5*2)=10/10)=1
(1/2)(1/3)=1/6
(-1/4)(-2/3)=[(-1)*(-2)] / (4*3)=2/12=1/6
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