a)
[tex] \binom{2x + y = 20}{x - 3y = - 4} [/tex]
[tex] \binom{y = 20 - 2x}{x - 3(20 - 2x) = - 4} [/tex]
[tex] \binom{y = 20 - 2x}{x - 60 + 6x = - 4} [/tex]
[tex] \binom{y = 20 - 2x}{7x = 56 | \div 7| } [/tex]
[tex] \binom{y = 20 - 2x}{x = 8} [/tex]
[tex] \binom{y = 20 - 2 \times 8}{x = 8} [/tex]
[tex] \binom{y = 20 - 16}{x = 8} [/tex]
[tex] \binom{y = 4}{x = 8} [/tex]
b)
[tex] \binom{5x + 3y = 12}{2x - y = 7}[/tex]
[tex] \binom{5x + 3y = 12}{- y = 7-2x |\div (-1)}[/tex]
[tex] \binom{5x + 3y = 12}{y = -7+2x}[/tex]
[tex] \binom{5x + 3(2x-7) = 12}{y = -7+2x}[/tex]
[tex] \binom{5x + 6x-21 = 12}{y = -7+2x}[/tex]
[tex] \binom{5x + 6x= 12+21}{y = -7+2x}[/tex]
[tex] \binom{11x = 33|\div 11}{y = -7+2x}[/tex]
[tex] \binom{x=3}{y = -7+2*3}[/tex]
[tex] \binom{x=3}{y = -7+6}[/tex]
[tex] \binom{x=3}{y = -1}[/tex]