>>309
y = (-5x+18)/2 = -(5/2) x + 9
これを
13x+3y = 49
へ代入すると
13x + 3 { -(5/2) x + 9 } =49
13x - (15/2)x + 27 = 49
{13-(15/2)}x + 27 = 49
(11/2)x = 49-27
(11/2)x = 22
11x = 44
x = 4