because the y is opposite of each other in both equations you can cancel them out
3x=28
3x=14
next you add 3x to 3x, then 28 to 14 and get...
6x=42
next divide both sides by 6 and you get your answer for x...
x=7
now you must solve for y and you do this by substituting what you found for x in one equation it doesn't matter which because you'll get the same answer but you can check for both to make sure...
by deduction, 5 and 7 are prime numbers and 15 is not a multiple of 70, therefore the answer must be 10 as it is a non-prime number and is a factor of 70 (I.e. 10 and 7)