2s-13t=120, -14s+91t=-840

If you multiply the first equation by 7, you will get exactly the second equation.

So this system of equation has an infinite number of solutions. Geometrically, it means that two straight lines are coincident, so there are infinite points of intersection between the lines.

The solution is:
s=(120+13p)/2
t=p
where p=any real number.