Question

980 - х + 930 - х = 890

Answer 1

980 - х + 930 - х = 890
1910-2х=890
2х=1910-890
2х=1020
х=1020:2
х=510

Проверим:
980-510+930-510=890
890=890

Answer 2

-x-x=890-980-930
-2x=-1020
x=-1020:(-2)
x=510