Algebra
posted by Sara .
8s+ 12 = 16, 6s+ 9t = 12 solve by substitution

I think you have a typo and you should have 12t in the first equation
If so, then drwls made the following observation
http://www.jiskha.com/display.cgi?id=1282324238 
8s+12=16............. equation 1
6s+9t=12............. equation 2
Multiply equation 1 by 3 and equation 2 by 4 and subtract.
+24s+36= +48
+24s+36t=+48
   (sign will change when sub)
.............
3636t=0
t=1
put t=1 in equation 2
6s+9(1)=12
s=1/2
CHECK 6(1/2)+9(1)=12
12=12
LHS=RHS so your answer is correct