-2in = the equals sign has a slash through it 0
There's only information to answer question C.
We can only know that 32 marbles were chosen. (yellow and black)
Why?
To answer the first two questions (a and b), we need to know the total number of marbles, so, we can only answer the last question (c).
We know that 7/10 of the 20 yellow marbles were chosen.
So, calculating we have:

Also, we know that 18 black marbles were chosen, so, the total of marbles chosen is equal to 32 (yellow marbles and black marbles)

Have a nice day!
The solution (x,y) needs to work in both equations. With multiple choice you can do guess and test method. Otherwise you need to combine the two equations into one by substitution or elimination methods.
21. substitution works well here since one equation is y= x+3. you can easily substitute the (x+3) for y in the other equation.
2x + (x+3) = -6
3x + 3 = -6
3x = -9
x = -3
use this in either equation to find y.
y = (-3)+3
solution: (-3,0)
22. these both equal y so they equal each other.
4x - 1 = 3x +6
4x - 3x = 6 + 1
x = 7
use this in either equation to find y
y = 4(7) - 1
y = 28 - 1
y = 27
solution: (7,27)
23. one equation is y = 2x - 3. So substitute (2x -3) for y in the other equation.
4x = 2(2x - 3) + 6
4x = 4x - 6 + 6
4x = 4x
x = x
any solution will work, infinitely many.
24. infinitely many solutions means the two equations are the same equation. Like the previous one they didn't look the same but if you put them both in terms of y...
y= 2x-3
4x = 2y + 6
4x - 6 = 2y
(4x - 6)/2 = y
2x - 3 = y
same equation.
from the image, I can't see all options but A & C do not look like the same equations. It's got to be either B or D.