What is the formula of the angle between two straight lines?

What is the formula of the angle between two straight lines?

The formula of angle between two straight lines is as follows
tgθ=(k2-k1)/(1+k1*k2)
K1 and K2 are the slopes of two straight lines respectively
The slope formula of straight line: k = (y2-y1) / (x2-x1)

[higher slope] given the slope of the line k = 2, a (3,5), B (x, 7), C (- 1, y) are the three points on the line, find the value of X and y
Given that the slope of the line k = 2, a (3,5), B (x, 7), C (- 1, y) are the three points on the line, find the value of X and y
PS: the answer copied from the Internet is ignored directly

k=2
y=2x+b
Then 5 = 6 + B
b=-1
So y = 2x-1
Then 7 = 2x-1
y=-2-1
So x = 4
y=-3

How to determine x1y1x2y2 in the formula for calculating the slope of a straight line
Given the linear slope formula k = (y2-y1) / (x2-x1) and any two points a and B on the line, according to their x, Y values, how to determine which is X1 Y1, X2 Y2?

Guo Dunyong replied: if the coordinates of two points are a (A &;, B &;) and B (A &;, B &;), it is customary to determine X1 = A &;, Y1 = B &;; x2 = A &;, y2 = B &;. However, it can be the opposite. Instead, we can determine X1 = A &;, Y1 = B &;; x2 = A &