Question 1141351
f(x+5) = x + 12 = (x+5)+7  -->  {{{ highlight( f(x) = x+7 ) }}}
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To see this more clearly, use a different letter for the independent vsriable, then set it equal to x+5 as a 2nd step:<br>

f(w) = w+7<br>

Now set w=x+5:<br>

f(x+5) = (x+5)+7 = x+12