Question 1155522
An unknown quantity can be any letter or symbol, so
{{{ m }}} is perfectly OK to use.
Don't think in terms of "moving" things to the left or right.
Just about the only rule you need is:
" Whatever you do to one side of an equation, you must do
the same thing to the other side."
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You want to end up defining what a positive {{{ m }}} is,
not a negative {{{ m }}} is, so your first step is "Add {{{ .5m }}}
to both sides."
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{{{ -.5m - .75 = 2.15 }}}
{{{ .5m = .5m }}} 
Now add these equations
{{{ 0  - .75 = .5m + 2.15 }}}
Now subtract {{{ 2.15 }}} from both side
{{{ 0  - .75 = .5m + 2.15 }}}
{{{ -2.15 = -2.15 }}}
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{{{ -2.15 - .75 = .5m }}}
Sum the terms on the left side
{{{ -2.9 = .5m }}}
Divide both sides by {{{ .5 }}}
{{{ -5.8 = m }}} answer
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Now you can check the answer:
{{{ -.5m - .75 = 2.15 }}}
{{{ -.5*( -5.8 ) - .75 = 2.15 }}}
{{{ 2.9 - .75 = 2.15 }}}
{{{ 2.15 = 2.15 }}}
OK
Hope this helps