x = - 2
y = 4
7x - 4y = -30
3x + 4y = 10
add
7x + 3x - 4y + 4y = - 30 + 10
10x = - 20
x = - 20 : 10
x = - 10
replace x
3(-2) + 4y = 10
- 6 + 4y = 10
4y = 10 + 6
4y = 16
y = 16 : 4