To determine which equation Tillie is solving after subtracting 8 from both sides, let's analyze each option:
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\(12 = -x - 8\): If we add 8 to both sides, we get \(12 + 8 = -x\) or \(20 = -x\).
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\(12 = -x + 8\): If we subtract 8 from both sides, we get \(12 - 8 = -x\) or \(4 = -x\).
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\(12 - x = -8\): If we add \(x\) to both sides and then add 8 to both sides, we would not be left with a straightforward situation that results in \(8\) (the left side would have to become 8).
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\(12 - x = 8\): If we subtract 8 from both sides, we get \(12 - 8 = x\) or \(4 = x\).
Since Tillie's first step is to subtract 8 from both sides of the equation, we can conclude that the equation she is solving is \(12 = -x + 8\). This is because subtracting 8 aligns with the process that she is described as performing.