Question

para amanhã!!
3X-Y=15
X+4Y=21

Answer 1

3x - y = 15
      y = 3x - 15 .. subtitution

x + 4y = 21
x + 4 (3x-15) = 21
x + 12x-60 = 21
13 x = 81
    x = 81/13
      

81/13 + 4y = 21
       4y = 21-81/13
            = (21*13 - 81)/13
            =  192/13
         y = 192/4*13
           = 48/13

Answer 2

$\left \{ {{3x-y=15} \atop {x+4y=21}} \right. \\ \\ \left \{ {{3x-y=15} \atop {3x+12y=63}} \right. \\ \\ Subtraindo\\ \\ -13y=-48\\ \\ y=\frac{48}{13}\\ \\ 3x-\frac{48}{13}=15\\ \\ 39x-48=195\\ \\ 39x=243\\ \\ x=\frac{243}{39}=\frac{81}{13}$