If 283 \(\equiv\) 3 \((mod \ 28)\) and 819 \(\equiv\) 7 \((mod \ 28)\), then find the subtraction and multiplication in modular arithmetic.
1. −536 \(\equiv\) \((mod \ 28)\)
2. \(\equiv\) 21 \((mod \ 28)\)
(Note: Insert a negative sign wherever necessary.)