Find the solution of the system for which x 1 equals 0 and s 1 equals 0. 3 x 1 plus 7 x 2 plus s 1 equals 49 x 1 plus 4 x 2 plus s 2 equals 31

It looks like you're trying to solve
  3x₁ + 7x₂ + s₁ = 49
  x₁ +4x₂ +s₂ = 31

Fill in the given values for x₁ and s₁ and your system reduces to
  7x₂ = 49
  4x₂ +s₂ = 31
The first of these equations tells you x₂ = 49/7 = 7, so the second equation becomes
  28 +s₂ = 31
and it is immediately obvious that s₂ = 3

Your solution is (x₁, x₂, s₁, s₂) = (0, 7, 0, 3).