Look at the LHS: you have the square of x, then you have 2 times x, times -1. to complete a square you just need to add the square of -1, that is 1. That becomes
To find the equation take the slope 2/3
y-y(1)=m(x-x(1)
y-4=2/3(x-8) = 2/3x-16/3
Next solve for y
y=2/3x-16/3 +4(turn 4 into a fraction with a common denominator)
y=2/3x-16/3 + 12/3
y=2/3x-4/3
