3−7x2=12\displaystyle {3-7x \over 2} =1223−7x=12
3−7x=2⋅123-7x=2\cdot123−7x=2⋅12
3−7x=243- 7x = 243−7x=24
−7x=24−3-7x = 24-3−7x=24−3
−7x=21-7x = 21−7x=21
x=−3x=-3x=−3
Ответ: -3