Question 820966
You have to fix the foundation of your algebra
Maybe the way the question is being asked is
confusing. 
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You are correct that {{{ y - 7 }}} is an expression
When you add on ( = something ) then it
becomes an equation.
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I can make this expression equal to anything I want.
Suppose I say:
{{{ y - 7 = 99999 }}}
I must isolate {{{ y }}} on the left side, so I will add {{{ 7 }}}
to both sides:
{{{ y - 7 + 7 = 99999 + 7 }}}
{{{ y = 100006 }}}
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Like I said, I can put anything I want on the right side, but
it doesn't necessarily have to be true
Suppose I said:
{{{ y - 7 = y - 6.999 }}}
It's just not a true statement, and there's no way to make it true
The following is true, but {{{y}}} can be anything, not just a single number:
{{{ y - 7 = y - 7 }}}
If I say {{{ y = -2972745196243 }}}, that will work,
and so will {{{ y = 837200106911153 }}} or {{{ y = 1 }}}
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Hope this helps