Which equation in point slope form represents a line with m=-15 that goes through the point (7,-1)

The answer is:  " y = -15x + 104 " .

  y = mx + b ;

 m = -15

 y  = -15x + b

When x = 7 ; y = -1 ; 

-1 = -15(7) + b

-1 = -105 + b ;

Solve for "b" 

  -1 = -105 + b ;  ↔  - 105 + b = -1 ; 

↔  b − 105 = -1  

Add "105" to each side of the equation:

    b − 105 + 105 =  -1 + 105 ;

    b =  104

   y = mx + b ;   →   y = -15x + 104 .

Idon’t know actually i’m in 5th grade and don’t know