Verified answer

1) 21-4d=42+3d

-4d-3d=42-21

-7d=21

d=-3

2)5y-9= -81-4y

5y+4y= -81+9

9y= -72

y= -8

3) 3(c+3)+21=60

3c+9+21=60

3c+30=60

3c=30

c=10

4)24-3(a-8)=18

24-3a+24=18

48-3a=18

-3a = -30

a=10