Gus crossword puzzle

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Across
  1. 5. slope=0
  2. 6. replacing letters with numbers
  3. 7. where a line crosses y-axis
  4. 9. all natural numbers and zero
  5. 12. slope=undefined
  6. 14. 2 terms
  7. 15. 1 term
Down
  1. 1. where a line crosses x-axis
  2. 2. starting point
  3. 3. 3 terms
  4. 4. it passes the vertical line test
  5. 8. term with no variable
  6. 10. Ax+By=C
  7. 11. lines are horizontal
  8. 13. letter representing a value