unknown letter which an equation has to be solved for

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Across
  1. 1. equation with x with exponent 1
  2. 4. to substitute
  3. 6. to take a term and change the side
  4. 7. the term without letter
  5. 8. generally the number of solutions of a linear equation
Down
  1. 2. when the equation has no results
  2. 3. letter which an equation has to be solved for
  3. 5. the name of the side before the =