m represents the slope of a straight line

where (x1,y1) is a point by which this straight line passes through.
according to our givens in here this point is (3,4) (by substituting in the previous equation)

(multiplying both sides by (x-3))
2(x-3)= y-4
2x - 2*3 = y - 4
2 x- 6 = y - 4 (by adding 4 to both sides)
y - 4 +4 = 2x -6 +4
y = 2 x -2