Substitute.
2x+(2x -15) = -3
4x -15 = -3
Add 15 to both sides
4x = 12
Divide by 4
X=3
2(3) + y = -3
6+y=-3
-6 from both sides
Y=-9
(3,-9)
For this case we have the following function:

The first thing you should do is graph the function to see the behavior.
When observing the graph (in the attached image) we observe that the following point belongs to the graph:

To prove it, let's evaluate the point in the function:

The equation is fulfilled and therefore the point belongs to the graph.
Answer:
(6.2, 7.4)
See attached image
Answer:
x = -2, y=1
Step-by-step explanation:
x-7y=-9---------------------equation 1
-x+8y=10-------------------equation 2
From equation 1, make x the subject of formula
x=7y-9----------------------equation 3
substituting x=7y-9 in equation 2,
-(7y-9)+8y=10
Expanding bracket
-7y+9+8y=10
Collecting like terms
8y-7y=10-9
y=1
substituting y=1 in 3
x=7(1)-9
x=7-9
x=-2
I think the answer to your question is 15 hope this helps