Solve the system: {15x - 9y = 0; 2x + 3y = 21}

from (1)...
y=5/3 x

from (2)...
2x+3(5/3 x) = 21
2x+5x=21
7x=21
x=3

then
y=5/3 (3)
y=5
solution:
(3,5)