Solve by using substitution
6x - y = 78
5y = x

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

B. because the 7 represents "seven times", x represents "a number", 12 represents "plus twelve", and -4 represents "minus four"