substitution method of s-t = 3
                                       s/3 + t/2 = 6
 

S-t =3 ..(1)
s/3+t/2=6 ...(2)
s=3+t....(3)
substitute the value of eq.3in eq2
s/3+t/2=6
3+t/3+t/2=6
(3+t) multiply 2/3 multiply by 2 +t multiply by 3/2 multiply by 3=6
6+2 t+3 t/6=6
6 + 5 t =36 
5 t =36-6
5 t = 30 
t = 30 / 5 = 6
t =6 
put the value of t in eq. in 3
S = 3 + t
S = 3 + 6= 9
S = 9