Final answer:
To convert y-7 into 7-y, one must multiply y-7 by -1.
Step-by-step explanation:
The question asks By what must y-7 be multiplied to obtain 7-y.
To convert y-7 into 7-y, we need to multiply by -1 because multiplying by -1 will change the signs of both terms in the expression, effectively swapping their places and making y-7 become 7-y. This is because -1 × y equals -y and -1 × -7 equals positive 7.