Question

8s+ 12 = 16, 6s+ 9t = 12 solve by substitution

Answer 1

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

Answer 2

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)
.............
36-36t=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