Need ! can't figure out question 2 a and b and 3 a and b. plz with !

2 (a): Lena's method is correct 

 (b) Explanation: - We can explain that she was correct by checking the equation. In order to check that, we have to put the values of x and y.

4y = 3x + 7
Substitute for y = 10, x = 11  [ Lena's answers ]
4(10) = 3(11) + 7
40 = 33 + 7
40 = 40  [ L.H.S. = R.H.S ] 

Now, checking with 2nd equation, 
9x + 4y -139 = 0
9(11) + 4(10) - 139 = 0
99 + 40 - 139 = 0
139 - 139 = 0
0 = 0 [ L.H.S. = R.H.S ] 

As L.H.S. is equal to R.H.S. for both equation, She was correct

3)  a) System of equations would be:
  x + y = 1
  x - y = 11

b) Solving the equation, 
Take 2nd equation, 
x - y = 11
x = 11 + y

Now, substitute it in 1st equation., 
x + y = 1
11 + y + y = 1
2y = 1 - 11
y = -10 / 2
y = -5

Substitute it in 2nd equation, 
x - y = 11
x - (-5) = 11
x + 5 = 11
x = 11 - 5
x = 6

In short, x is equal to 6, and y is equal to -5

Hope this helps!