They equal different things. If you simplify the second equation, it is 15x-4 = 15x-12 because of the 3 outside of the paranthesis. Therefore, the equation is wrong.
(a) m = 
calculate the slope (m) using the ' gradient formula '
m = 
let (x₁, y₁) = (8, 9) and (x₂, y₂) = ( - 2, 4)
m =
=
= 
(b) y - 9 =
(x - 8)
the equation of a line in ' point- slope form ' is
y - b = m(x - a)
where m is the slope and (a, b) a point on the line
choose any of the 2 given points for (a, b) → (8, 9)
y - 9 =
(x - 8) → " point- slope form"
(c) y =
x + 5
the equation of a line in ' slope-intercept form ' is
y = mc + c
where m is the slope and c the y-intercept
expand and simplify the point- slope equation
y - 9 =
x - 4
y =
x - 4 + 9
y =
x + 5 → " slope- intercept form"
If youre trying to solve for the variables you have to solve for one of them in one equation first, so if you solved for x in 2x+3y=16 youd get x=8-(3/2)y. Then youd have to sub in that x for the x in the other equation so youd get 3(8-(3/2)y)-2y=13 then solve for y. Once you have the value of y it should be the same in both equations. Plug that new value of y into the equation of youre choosing then sove for x