( a + 7 ) : 6 - 552 : 23 = 25
( a + 7 ) : 6 - 24 = 25
( a + 7 ) : 6 = 25 + 24
( a + 7 ) : 6 = 49
a + 7 = 49 × 6
a + 7 = 294
a = 294 - 7
a = 287