Use substitution to solve the system.

x=3y-8
3x-4y=-9

x=3y-8
3x-4y=-9

We are told that:
x=3y-8

So replace x in 3x-4y=-9 with x=3y-8 like so: 
3(3y-8)-4y=-9 

Start by distributing the 3: 

Now you have: 

Combine like terms, now you have: 

add 24 to both sides 

Now you have:

Divide both sides by 5 to get: 
y=3 

Now that you know y=3 plug it into: 
x=3y-8
x=3(3)-8
x=1 

Final  
(1,3) 

Equation 1)  x = 3y - 8
Equation 2)  3x - 4y = -9

For equation 1, subtract 3y from both sides.

1)  x - 3y = -8

Then, multiply ALL of equation 1 by 3.

1)  3(x - 3y = -8)

1)  3x - 9y = -24
2)  3x - 4y = -9

Subtract equations from one another.

-5y = -15

Divide both sides by -5.

y = 3

Plug in 3 for y in the second equation.

3x - 4y = -9

3x - 4(3) = -9

Simplify.

3x - 12 = -9

Add 12 to both sides.

3x = -9 + 12

Simplify.

3x = 3

x = 1

~Hope I helped!~