y=7x
y+6x=39

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Substitute the first equation into the second equation.

Can you do it?

I hope you know what substitution means...

Equation 1: Y = 7X
Equation 2: Y + 6X = 39

Subtract -7X from BOTH sides, Eq. 1 becomes Y - 7X = 0

Next, multiply Eq. 1 by -1: -Y + 7X = 0

Now, we have Eq. 1 and Eq. 2:
Eq. 1 : -Y + 7X = 0
Eq. 2 : Y + 6x = 39

Adding the two equations above, the Y's cancel each other out:
13X = 39

Solving for X:
X = 39/13 = 3

Don't forget to check your answer:

Eq. 1: Y = 7X = 7*3 = 21
Eq. 2: Y + 6X = 21 + (6*3) = 21 + 18 = 39