Answer :

crisforp
12a / 84 + 21 / 84 = 70 / 84 ;
( 12 a + 21 ) / 84 = 70 / 84 ;
12a + 21 = 70;
12a = 49 ;
a = 49 / 12 ;
apologiabiology
a/7+1/4=5/6
a/7=4a/28
1/4=7/28
4a/28+7/28=(4a+7)/28
(4a+7)/28=5/6
multiply both sides by 28
4a+7=70/3=23 1/3
subtract 7 from both sdies
4a=16 and 1/3
divde both sides by 4
a=4 1/12