3y=18-5x 即 5x+3y=18①
3x=a+5y 即 3x-5y=a②
①×5+②×3得
(25+9)x=90+3a
34x=90+3a
x=(90+3a)/34
①×3-②×5得
(9+25)y=54-5a
34y=54-5a
y=(54-5a)/34
x=(90+3a)/34
y=(54-5a)/34