Get them so that the terms with s's and t's
are on the left side, and the terms without 
them are on the right side.

The first equation is already like that.
Get the second one that way by adding -2t to
both sides:

     5s = 2t+7
  5s-2t = 7



Now let's make the t's cancel out.

the coefficient of t in the 1st equation is +3
the coefficient of t in the 2nd equation is -2

We want to change the +3 and the -2 so they will
cancel.

They already have opposite signs.
The least common multiple of 3 and 2 is 6.
If we multiply the 1st equation by 2, the coefficient
of t will change from +3 to +6
If we multiply the 2nd equation by 3, the coefficient
of t will change from -2 to -6
And then +6 and -6 will cancel.

So let's multiply the 1st equation through by 2,
and the 2nd equation through by 3



Now we add the two equations term by term:

4s+15s=19s, +6t-6t=0, and -2+21=19, 
so we have



Divide both sides by 19, and get



Substitute s=1 into the first original equation:




Add -2 to both sides

Divide both sides by 3


Solution: (s,t) = (1,-1)

Edwin