I'm assuming that subtraction sign is mean to be an equals sign. If that is true than the answer is 403.75.
For this case we have that by definition, the equation of a line of the slope-intersection form is given by:

Where:
m: It's the slope
b: It is the cut-off point with the y axis
While the point-slope equation of a line is given by:

Where:
m: It's the slope
It is a point through which the line passes
In this case we have a line through:
(8,4) and (0,2)
Therefore, its slope is:

Its point-slope equation is:

Then, we manipulate the expression to find the equation of the slope-intersection form:

Therefore, the cut-off point with the y-axis is 
ANswer:

Answer:
29
Step-by-step explanation:
c² = 2² + 5²
c² = 4 + 25
c² = 29
c = sqrt(29)