There are two equations given in the question. Also there are two unknown variables in the form of "s" and "t". I hope you want to find the exact values of the two unknown variables in the question.
s = t + 2
And
2t + s = 17
Now let us put the value of s in terms of t from the first equation in the second equation.
Then
2t + s = 17
2t + t + 2 = 17
3t = 17 - 2
3t = 15
t = 15/3
= 5
Now let us put the value of t that we have already determined, in the first equation.
Then
s = t + 2
= 5 + 2
= 7
So the value of the unknown variable "t" is 5 and the value of the unknown variable "s" is 7.