How do I find all angles of triangle by finding slopes and use tangent formula? I'm new to this topic I'm having confusions

Question

Find the angles of triangle with vertices A(-4, -2), B(1, -1) and C(3, 4).

Answer 1

AB has slope = (-1+2)/(1+4) = 1/5 = 0.2

use a calculator with an inverse tangent function

tan^-1(.2) = 11.31 angle with the x axis

BC has slope = (4+1)/(3-1) = 5/2 = 2.5

tan^-1(2.5) = 68.20 angle with the x axis

angle ABC = 11.31 + (180-68.20) = 11.31 + 111.8 = 123.11 degrees

It helps to plot the 3 points and connect them with straight lines. Even a rough sketch would show that angle ABC is not acute

do the same method with the other two angles, which are acute and relatively small angles. Angles ACB and CAB are both less than 45 degrees.

slope of AC is (4+2)/(3+4) = 6/7

tan^-1(6/7) = 40.6 degree angle with the x axis

BC has 68.2 degree angle with the x axis

angle ACB = 68.2 - 40.6 = 27.6 degrees

AC has angle 40.6 degrees with the x axis

AB has angle 11.31 degrees with the x axis

Angle CAB = 40.6-11.31 = 29.29 degrees

27.6 + 29.3 + 123.1 = 180

angles of any triangle sum to 180 degrees