Find angle relative to horizontal y-line with two given points

Hi! I’m sure this is a simple math equation but I’m having trouble figuring it out.
Im trying to figure out a formula to identify the angle marked red, in relation to the horizontal line y. The two points A and B is known.

findAngleQuestion

local y = y2 - y1
local x = x2 - x1
local theta = math.atan2(y, x)

(Assuming X is the horizonal axis.)

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Let me just delete the draft I was doing because I was making this way more complicated then it needed to be lmao. So tired I forgot about atan2 and the other math functions.

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@skilpus3000

For explanation math.atan2 is essentially the math.atan function (arctangent; inverse tangent; etc) except for that it handles for the introduction of the negative quadrants.

If your unfamiliar with what the tangent function is, it is essentially a trigometric function.

Tangent of theta is the opposite side length over the adjacent side length, therefore the inverse tangent of the opposite side length over the adjacent side length is theta (the angle)!

image

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