Question

resolva =
{x+y=21
{y=x/2+3

Answer 1

{x+y=21 -> x=21-y
{y=$\frac{x}{2} +3$

y=$\frac{x}{2} +3$

y=$\frac{21-y}{2} +3$

(mmc:2/1*y=2y ;  2/2*21=21 ; 2/2*y=y  ; 2/1*3=6

$\frac{2y}{2}= \frac{21-y}{2} + \frac{6}{2}$

cortar o denominador:
2y=21-y+6
2y+y=21+6
3y=27
$y= \frac{27}{3}$
y=9


x+y=21
x+9=21
x=21-9
x=12