Find the Measure of Angles A & C in Triangle ABC

[kidia]

Using the formula relating the tangent of angle p enclosed between two given straight line L1 and L2 and respective slope m1 and m2,Find the measure in degrees of angles A and C of the triangle with vertices A(-3,4),B(5,6) and C(2,1)

My work is
Tan A = Ma = yb-ya/xb-xa=6-4/5+3=2/8=1/4
Tan A =1/4

Tan C = Mb = yb-yc/xb-xc=5/3
Tan C = 5/3

So Angle A = arctan 1/4
Angle C = arctan 5/3

Any idea on this?

 

[HallsofIvy]

Those are the slopes of the lines and so give the angle each line makes with the horizontal.
For example, you have calculated, correctly, that the line from B to C makes an angle with the horizontal of arctan(5/3) and that the line from A to C makes an angle with the horizontal of arctan (5/3). Now draw a picture. You should see that the horizontal line through C makes 3 angles. The center is the angle C in the triangle. The arctangents are the other two angles. Use the fact that the total angle is 180 degrees (or pi radians).

 

[kidia]

yah I got u'r point