Question 302008
s+t=-5 ... equation 1
s-t=3 ... equation 2

Rearrange equation 2: 
s-t = 3
s = t + 3 
Put s = t+3 into equation 1 

(t+3)+t = -5
3 + 2t = -5
2t = -8
t = -4

Put t = -4 into equation 2 
s -(-4) = 3
s + 4 = 3 
s = -1 

Answer: s = -1 and t = -4