Question 824661
first find a slope:

Slope {{{m = (y1-y2)/(x1-x2)}}} 

given: {{{x1=0}}} and {{{x2=a}}}, {{{y1=a}}} and {{{y2=2a}}},

{{{m = (a-2a)/(0-a)}}}

 {{{m = (-a)/(-a)}}}

{{{m =1}}}

so,
 {{{y-intercept = a}}} because is given point P(0,a), menas  if {{{x=0}}} => {{{y=a}}}

 --> {{{y = x + a}}}