Question

Anna and Belle are asked to solve –2x – 15 = 6x + 9. Identify where one of them made an error.


Anna
Belle
–2x – 15 = 6x + 9
–2x – 15 –6x = 6x + 9 –6x
–8x – 15 = 9
–8x – 15 + 15 = 9 + 15
–8x = 24
Negative 8x over negative 8 equals 24 over negative 8
x = –3
–2x – 15 = 6x + 9
–2x – 15 + 2x = 6x + 9 + 2x
–15 = 8x + 9
–15 + 9 = 8x + 9 + 9
–6 = 8x
Negative 6x over 8 equals 8x over 8
Negative two-thirds

Answer 1

1
Add 2x to both sides
−15=6x+9+2x

2
Simplify 6x+9+2x to 8x+9
−15=8x+9

3
Subtract 9 from both sides
−15−9=8x

4
Simplify −15−9 to −24
−24=8x

5
Divide both sides by 8
−248=x

6
Simplify 248 to 3
−3=x

7
Switch sides
x=−3