you want to get two of the letters with the same number next to them ie you want the x's to be the same, so you find a number that both 4 and 3 go into- which is 12. you need to multiply four by three to get 12 and you need to multiply 3 by 4 to get 12. so multiply the whole of the first equation by 4 to get 12x + 8y = 36 then the whole of the second equation by 3 to get 12x - 18y = 75 then you subtract the first equation from the second (or the other way round it doesn't matter) to cancel out the x's to leave -26y = 39
Mark your explanation to poor Mary is quite difficult to follow written in that style.
Plus Mr Jameson (the Administrator) asked her to let us know what she knew first;
so that was a little impertinent of you to boomerang in as you did!
12x + 8y = 36 is correct, but it should be 12x - 18y = 75 rather than "-12x + 18y = -75" you got the symbols mixed up.
so now you've got the two x values the same you can cancel them out by subtracting one of the equations from the other one eg
12x - 18y = 75 then take away
12x + 8y =36 from that
leaving you with -26y = 39
so y = - 1.5
Here's my explanation,
Multiply (i) by 3,
adding (i)(a) and (ii), we get,
dividing by 13 on both sides, we hv,
Substitute for x in any one of the equations, say eqn (i),
3(4) + 2y=9
or y=-3/2 or -1.5
this is ur soln set (4,-1.5)
All Mary needed to do was *add* her two equations.
What you get is 26y = -39.
I fear that what she wrote was 26 = -39.
All she did wrong was lose her y.