How do i solve this with substitution?

They give you an "x =" statement so you plug in the right side of the equals sign into the first equation wherever x appears. So...

y = 10 - 2(y - 4)
y = 10 - 2y + 8
y = 18 - 2y
y + 2y = 18
3y = 18
y = 6

Now we plug in this result for the y-value into the "x =" statement.

x = 6 - 4
x = 2

Your answer is (2, 6) 

Yes i could be the length of the triangle

I’m confused as to what you are asking 66 2/3 percentage wise