You are given an equation of a line and a point and told to use substitution to determine whether the point is on the line

Fine ! 

-- Take the 'x' of the point and write it in place of 'x' in the equation. 
-- Then take the 'y' of the point and write it in place of the 'y' in the equation. 
-- Now the equation has nothing but numbers in it, and it says that the numbers
   on the left side are equal to the numbers on the right side. 
-- Look at it and see if the statement is true. 
-- If true, then the point is on the line. 
-- If false, then the point is not on the line.

Examples:

1).  Equation . . .  y = 6x + 1
      Point. . . . . . . (3, 19)            (x=3, y=19)

Plug the point into the equation:       y = 6x + 1
                                                          19 = 6(3) + 1
                                                          19 = (18) + 1
                                                          19 = 19
The statement is true.
The point is on the line.

2).  Equation . . .  2x + 3y = 9
      Point . . . . . . . (1, 2)                   (x=1, y=2)

Plug the point into the equation:       2 x   + 3 y   = 9
                                                             2(1) + 3(2) = 9
                                                               2    +  6    = 9
                                                                     8        = 9
The statement is false.
The point is NOT on the line.