What is the slope and the y intercept of the line y=6x-9?

The slope is:  6 .
The y-intercept is:  -9  ;  (or, more precisely:  "(-9, 0)" .

   Note:  The line in written in "point-slope form"; also known as:
"slope-intercept format" ;  that is:  " y = mx + b"  ; in which:
                                              m = slope = coefficient of "x" ; 
                                              b = "y-intercept" (or, more precisely; the value
                                                  of "x" in the "x-coordinate" of the point of the 
                                                   "y -intercept).

    So:   Given:  y = 6x - 9 ;   m = slope = 6 ;
                                             b = -9 .

c = 22.3 km

Step-by-step explanation:

You can solve this problem using cosine law:

where a and b are the legs of the triangle, c is the unknown length and the angle gamma is the angle opposite length c. We know that

a = 50 km

b= 52.4 km

gamma = 25°

c^2 = (50 km)^2 + (52.4 km)^2 -2(50)(52.4)cos25°

= 496.707 km^2

or

c = 22.3 km