Step-by-step explanation:
-7(y-1)= 21
-7y+7=21
-7y=21-7
-7y=14
divide by-7
y=-2
The question only supplied us with the mass of the new baby which is:

This mass is too bizzarre & unrealistic for
The factors of 7 are 7, 1.
The factors of 12 are 12, 6, 4, 3, 2, 1.
12*7=84
7+5=12 so it can be one of these 8+4=12, 9+3=12, 10+2=12, and 11+1=12.