The rate of change is equal to rise over run
= (-1 - 8)÷(3 - 0)
= -9 ÷ 3
= -3
When you say initial value I'm assuming you mean the y-intercept
In that case you first need to find the equation of the line by substituting the gradient and point into the gradient intercept formula
y - 8 = -3(x - 0)
y = -3x + 8
The y-intercept is when x = 0 so,
y = -3(0) + 8
Therefore y = 8
