A battery is generally expected to become weaker over time, as a consequence of the normal use of equipment. The presence of a weak battery would be found by a routine check, and replaced by routine maintenance.
The fact that a battery is weak would be considered the result of routine wear and tear
Y=x-7 this is because you subtracted the x to both sides. Since y is a negative, you divide by -1 causing the -x to turn positive and the positive seven to be negative seven.
