A sequence has equal steps and runs -11, then one missing term, then -3. What is the missing term?

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The two known terms stand eight apart, so each of the two equal steps covers four, and rising four from eleven below zero lands at seven below zero. Combining the sizes eleven and three without their signs gives fourteen below zero, and applying the step downwards from -11 instead of upwards gives -15; neither value sits between the two known terms.