2x+5y=-8 and 6x+15y=k
Notice that when you multiply 2x by 3 you get 6x. And when you multiply 5y by 3 you get 15y. So when you multiply -8 by 3 you will get k which will be equal to -24.
Checking by elimination method:
Equation 1 : 3(2x+5y=-8) --------> 6x+15y=-24
Equation 2: 6x+15y=-24
If you subtract both the equations you will get 0=0 ( zero=zero) which is the property of a dependent system