In order to find where these two lines intersect you must solve them simultaneously x
3x +5y = 78.... (1)
2x - y = 0 ........(2)
by multiplying eq (2) by 2 and adding it to eq (1)
5(2x - y = 0) ..... (3)
10x - 5y = 0 ..... (3)
eq(3) + eq (1) : 10x + 3x +5y - 5y = 78 + 0
⇒ 13x = 78
⇒ x = 6