Может быть любым из них, кроме №2.
5)
0.4х=-0.44
х=-0.44÷0.4
х=-1.1
6)
1/3х+1/9х=2/7-5/7
3/9х+1/9х=-3/7
4/9х=-3/7
х=-3/7÷4/9
х=-3/7×9/4=-27/28
7)
12а+15=17а-5
17а-12а=15+5
5а=20
а=4
4)
номер 4
<span>21,35 × 10^(−6)</span><span> = </span><span>0,00002135</span>
... = (y(4x - 3y) + x² + 3y²)/xy = (4xy - 3y² + x² + 3y²)/xy = (x² + 4xy)/xy = (x + 4y)/y = 4 + x/y.