Answer: the number must be placed in the box is 9
suppose: the number must be placed in the box is x
have:
5u + 3(4u - 6) = -3u -27 + 20u + x
⇔ 5u + 12u - 18 = -3u - 27 + 20u +x
⇔ 17u + 3u - 20u = -27 + 18 + x
⇔ 0 = x - 9
⇔ x = 9
Step-by-step explanation:
