(630-х)+210=500 ; (х+190) -370=330

(630-х)+210=500  
630 - х = 500 - 210
630 - х = 290
х = 630 - 290
х = 340                
(630 - 340) + 210 = 500
290 + 210 = 500
500 = 500

(х+190) -370=330
х + 190 = 330 + 370
х + 190 = 700
х = 700 - 190
х = 510                  
(510 + 190) - 370 = 330
700 - 370 = 330
330 = 330

(630-x)=290
x=630-290
x=340

(x+190)=700
x=700-190
x=510