3a+4b=-25

2a-3b=6

I assume we are solving ?

first times 3 ---> 9a + 12b = -75
2nd times 4 ---> 8a - 12b = 24
add them

17a = -51
a = -3

back into the first:
3(-3) + 4b = -25
4b = -16
b = -4

a = -3 , b=-4