You don't need to know much about trig here. You only need to know that the other acute angle in the triangle is less than 55°, so the measure of side x will be less than 8.9. Only one answer choice qualifies:
x ≈ 6.2
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If you actually want to do the trig, as opposed to simply rejecting unreasonable choices, then you want to make use of the tangent relation.
