Please help me

I'm not certain that this is the easiest way, but ...

I'm going to find the inverse of y  =  3x + b

--->  x  =  3y + b

--->  x - b  =  3y

--->  y  =  (x - b)/3

Since they intersect at the point (-3, a), set the two equations equal to each other:

--->  3x + b  =  (x - b)/3

--->  9x + 3b  =  x - b

--->  8x  =  - 4b

Since x = -3 at that point:

--->  8(-3)  =  -4b

--->   -24  =  -4b

--->  b  =  6

Original equation is y  =  3x + 6

At the point (-3, a) --->  a  =  3(-3) + 6   --->   a  =  -9 + 6   --->   a = -3