1. **Stating the problem:** We need to check if the given equations are written correctly. 2. **Equation a:** $15 - 6 = 15 + 6$ 3. **Check equation a:** Calculate both sides: Left side: $15 - 6 = 9$ Right side: $15 + 6 = 21$ Since $9 \neq 21$, equation a is **not correct**. 4. **Equation b:** $21 \div 7 = 21 - 7$ 5. **Check equation b:** Calculate both sides: Left side: $21 \div 7 = 3$ Right side: $21 - 7 = 14$ Since $3 \neq 14$, equation b is **not correct**. **Final answer:** Both equations a and b are written incorrectly.