Solving systems of equations

my goal here is to solve the system with the substitution method... but as soon as they threw fractions into the mix im lost

The problem im stuck on is:

Key: 1/2=fraction not division

X+Y=12
X= -3/2y+3/2

so by theory of substitution the problem becomes

-3/2y+3/2+y=12

ive gotten that far and now im frozen ive tried so many different things and every time i try i come up with Y^2=9 1/2 which obviously is wrong

can somebody please explain to me how to complete this problem (not just some kid looking for the answer i actually want to know HOW to complete the problem for future reference so please dont just give me the answer)

its the fractions and the signs that are throwing me off before they threw fractions into the mix i was able to do the substitution method fine with whole numbers

Thanks in advance 10 points for any1 who helps me understand how to do these type of problems

Here is a suggestion.
Given that you seem to not like fractions.
if you have an equation (something that has an equals "=" in it)
you can ALLWAYS simplify it by multiplying it by the least common factor of all the denominators (or if you prefer the least common denominator)
X+Y=12
X= -3/2y+3/2
Becomes
X+Y=12
2X= -3y+3
~~~~~~~~~~~
Now the hint. For substation you will always have an isolated variable(one all by itself). When there is not any, then you will use the addition method, but that comes next in you class
Take the top equation and move the y to the other side
X = 12-y
You have your isolated variable
Plug it into the OTHER equation (substitute it)
2X= -3y+3 becomes
2(12-y)= -3y+3
Now you have one equation and one unknown
So it can be solved
24 – 2y = -3y +3
21 = -y
Y =- 21
Plug back into ANY of the equations
X+Y=12
X + (-21) = 12
X = 33

12 - y = (-3/2) y + 3/2
24 - 2y = - 3y + 3
y = - 21

x = 12 + 21
x = 33

x = 33 , y = - 21