6x - y = 78... u know that x equals 5y so where ever u see a x.. substitute it with 5y...
SO FOR EXAMPLE:
-6(5y) - y = 78..... u get -30y - y = 78....
Finally -30y - y = -31y.....
And now ur left with -31y=78....
Today find y... divide them both by -31.... and when u get that answer substitute it to wherever u see x