What is the equation of the line passing through (3,8) and (5,4)​

Using the "slope-intercept form">  y = mx + b,  where m is the slope, and b is the y-intercept.

We want to find m and b, which is 2 unknowns.  Therefor, we need 2 equations.  We can get those by using the 2 given points for x and y values.

Using (1,3), we get:  3 = m(1) + b

Using (8,5), we get:  5 = m(8) + b

Finally, solve this pair of equations using one of the standard methods