The equation is:
R - 8 = 4
So to get R all by itself, you can add+8 to BOTH sides of the equation. This will 'cancel' the -8 that is next to R.
R - 8 + 8 = 4 + 8
-8 + 8 = 0 so let's simplify:
R + 0 = 4 + 8
Simplify a little more:
R = 12
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