hi this is the worked out solution for your problem:
1a y=1 , x= 6
First, you substitude x into your first equation since x=5+y. This gives you 5+y=y=7. Then, you solved for y and then x.

2b. Similar to the first question, you need to make it so that x is on one side. Sicne x-2y=7, I can move -2y to the other side of the equation, giving me x=7+2y. Then i substitude x into the first equation and find y. Then x.

3a. This is a bit trickier. First you bring the x into one side of the equation which give you 2x=12-5y. Sincec 4 is a multiple of 2 (2x2 = 4) , this means I can put my third equation into the second equation , giving me 2(12-5y) +3y = -4 . Then i solve for y and then x.

If you have any other question, feel free to ask!