Diketahui sistem persamaan 4x+3y=23 dan 5x-7y=-25. Nilai -3x+6y adalah.... Help me please

4x + 3y = 23 20x + 15y = 115
5x - 7y = -25 ==>20x - 28y =-100, (eliminasi persamaan)--> 43y = 215-->y= 5
(substitusi) 4x + 3y = 23
4x + 3.5 = 23
4x = 23-15
x = 8
didapat (x,y) = (8,5)
-3x+6y = -3.8 + 6.5 = -24 + 30 = 6
alhasil 6.