Question 864942
Since we know that (8,-3) is a solution we can substitute 8 for x and -3 for y

ax + by = 25 and 3ax - 5by = 3
8a -3b = 25,
3*8a - 5(-3)b = 3

8a -3b = 25
40a-15b=125 multiply by 5
24a+15b=3
add them
64a-128
a=2

8a -3b = 25
plug in a=2
16-3b=25
-3b=9
b=-3
a=2