21х-8х-3х=21+18-9 ; 10х=30/10; x=3
<span>21x-9-18-8x=21-3x</span>
<span>21х-8х+3х=21+9+18</span>
<span>16х=48</span>
<span>х=48:16</span>
<span>х=3</span>