Subtract 8d from both sides
21 = 12d + 5 - 8d
Simplify 12d + 5 - 8d to 4d + 5
21 = 4d + 5
Subtract 5 from both sides
21 - 5 = 4d
Simplify 21 - 5 to 16
16 = 4d
Divide both side by 4
16/4 = d
Simplify 16/4 to 4
4 = d
Switch sides
d = 4
Check answer
8d + 21 =12d + 5
Let d = 4
8 × 4 + 21 = 12 × 4 + 5
Simplify 8 × 4 to 32
32 + 21 = 12 × 4 + 5
Simplify 12 × 4 to 48
32 + 21 = 48 + 5
Simplify 32 + 21 to 53
53 = 48 + 5
Simplify 48 + 5 to 53
53 = 53