given that
3m -9n =24
when n= -1
substitute it in the equation
thus
3m -9(-1)=24
solving for m
3m +9 =24
3m =24-9
3m=15
dividing through by 3
3m/3 =15/3
m =5 . thus when n= -1 , m=5
also put n=1 into the equation
3m -9(1) =24
3m -9 =24
3m= 24+9
3m =33
dividing through by 3
3m/3 = 33/3
m=11
thus when n= 1, m=11
lastly, put n= 3 into the equation
3m -9(3) =24
3m -27 = 24
3m = 24+27
3m =51
dividing through by 3
3m/3 =51/3
m =17. Thus when n = 3 , m= 17.