It
can be described useing two other undefined terms :- point & line
Differences:
point slope form of line:
(a) equation is 
where m is slope and (x1,y1) is a point
(b)This equation of line can be determined when two points are given
(x1, y1) and (x2,y2)
slope intercept form of line:
(a) equation is 
where m is a slope
b is y-intercept
(b)This equation of line can be determined when y-intercept and slope are given
Similarilties:
(a) both are equation of straight line
(b) slope is always constant in both
-45 is the answer to 2+9-56+
Yes the equation is linear because the linear equation is y=mx+b and y=85x-600 is linear.