Question

what to do now....subtract or add....????sign will change or not...????

Answer 1

if we subtract signs will change
but by adding it won't change

Answer 2

Heya !!!

This is Your Answer...

Given :
6u + 4v = 24 (i)
6u + 9v = 39 (ii)

To solve it, we have to cancel out one value..
For this, we will subtract equation (i) from (ii), 
6u + 9v = 39 
6u + 4v = 24
(-)  (-)      (-)
-----------------
        5v = 15
v = 3.
Putting v=3 in equation (ii), 
6u + 9X3 = 39 
6u = 12
u = 2.

Now, the Question arises why do we add? Why do we not subtract it?

Welll... We have to cancel the one value.. 
Lemme show you an example where we will add.. to cancel out..

Lets take..
2a + 3b = 6  (i)
a   - 3b  = 6  (ii)

Here we will add the two, and b gets cancelled. 
2a + 3b = 6  
a   - 3b  = 6
----------------
3a = 12
a = 4
and, b = -2/3

Hope You Got It