a= 1 3/5
3/4х =2/3+7/12
3/4x = 8/12 + 7/12
3/4x = 15/12
x = 15/12 * 4/3
x = 5/3
подставляет значение х во второе уравнение
1,4х+а=2(х+0,3)
1,4*5/3 + a = 2(5/3 + 0,3)
14/10 * 5/3 + a = 2(5/3 + 3/10)
14/6 + a = 2(50/30+9/30)
7/3 + a = 2 * 59/30
7/3 + a = 59/15
a = 59/15 - 7/3
a = 59/15 - 35/15
a = 24/15
a = 1 9/15 = 1 3/5