Question

2.5y + 3x=7 5x - 2.5y = 5

Answer 1

2.5y + 3x=7 (1st)
5x - 2.5y = 5 re-write -2.5y + 5x = 5 (2nd)

2.5y + 3x=7 (1st)
-2.5y + 5x = 5 (2nd)
---------------------------add
8x = 12
  x = 12/8 = 1.5

2.5y + 3x=7
2.5y + 3(1.5)=7
2.5y = 7 - 4.5
2.5y = 2.5
     y = 1

answer
solution (1.5 , 1)

Answer 2

If you put them together, +2.5y and -2.5y neutralize themselves while 5x and 3x become 8x. 

2.5y + 3x = 27
5x – 2.5y = 5

8x = 32
x = 4

2.5y + 3x4 = 27
2.5y + 12 = 27
2.5y = 27-12
2.5y = 15
y = 15/2.5
y = 6

X = 4 
Y = 6