a) 80 + 7 x a = 577
7 x a = 577 - 80
7 x a = 497
a = 497 : 7
a = 71
b) 1000 - a : 2 = 514
a : 2 = 1000 - 514
a : 2 =486
a = 486 x 2
a = 972
c) a : 3 + 383 = 507
a : 3 = 507 - 383
a : 3 = 124
a = 124 x 3
a = 372
d) ( 307 + a) : 8 = 107
307 + a = 107 x 8
307 + a = 856
a = 856 - 307
a = 549