2 ways to solve:
1) Plug in the answers...so, a=4, b=-1 is correct
2(4)+3(-1)=5
8+-3=5
5=5
2) Plug b=a-5 in 3b
2a+3b=5
2a+3(a-5)=5
———->so for 3(a-5)
3 x a=3a
3 x -5= -15
2a+3a-15=5
5a-15=5
+15=+15
————————
5a=20
—- ——
5 5
a=4
so, from here you can either look for the right answer and try to eliminate others OR you can then plug a=4, into b=a-5
b=a-5
b=4-5
b=-1
so a=4, b=-1 would make the first answer choice correct
Hope this helps!