We can to make the method of substitution.
Look:
3x + 15y = 35
Isolating 3x
15y = 35 - 3x
Dividing both the sides by 15
y = 35/15 - 3x/15
Simpliting:
y = 7/3 - x/5
Now let's equate this expression with the other
7/3 - x/5 = - x/5 + 7/3
0 = 0
Oh noOo!
There is solution infinatly to this question!
Because, the two equation are the same.
Hope this helps!