To write a equation of a line if you know two points that are on the line, you would first have to find the slope using the formula:
(y2 - y1) / (x2 - x2)
This would help you find the slope
You should always know that the standard form is y = mx + b
m = slope
plug in 2 of your points to the x and y values with your x like this:
(one y point) = (slope) (one x point) + b
This would help you to find the b of the equation
now you can write your equation like this:
y = (slope)x + (y - intercept you just found)
Here is a example using the points (1,2), (4,3)
Find your slope:
(3-2) / (4 - 1) =
1/3 = m(slope)
Now plug (I would be using points (1,2):
2 = 1/3(1) + b
2 = 1/3 + b
1 2/3 = b
Now you found your y - intercept:
y = 1/3m + 1 2/3