equations

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Across
  1. 2. mathematical way to say opposite
  2. 6. find the missing value
  3. 7. undo addition
  4. 8. do the same to both sides
Down
  1. 1. how you know if you are right
  2. 3. algebra with no equal sign
  3. 4. the missing value
  4. 5. maths with letters
  5. 9. Am I right?