Question

Find an equation of the line that passes through the given points. (Let x be the independent variable and y be the dependent variable.)
(3, 13) and (4, 20)

Answer 1

y=mx+b
13=m*3+b and the second equation gives
20=m*4+b
subtract first from second
m=7
then for b
13=7*3+b solve for b.

Answer 2

slope = (20-13)/(4-3) = 7

also for any (x,y)

slope = (y-20)/(x-4)

so

7 = (y-20)/(x-4)
7 x - 28 = y - 20
so
y = 7 x -8

Answer 3

Good, you have two different ways now :)