(19n-6n)÷4=26
(16m+5m)÷18=7
(17n-8n)×13=351

(19п-6п):4=26; 13п=26*4; 13п=104; п=104:13; п=8

(16м+5м):18=7; 21м=18*7; 21м=126; м=126:21; м=6

(17п-8п)*13=351; 9п=351:13; 9п=27; п=27:9; п=3