To solve these equations, we need to isolate the variable "x" in each equation. Let's solve them one by one:

a) a * x + 12 = 804
Subtract 12 from both sides:
a * x = 792
Divide both sides by "a":
x = 792 / a

b) 23 * x = 1242
Divide both sides by 23:
x = 1242 / 23

c) x / 34 = 78
Multiply both sides by 34:
x = 78 * 34

d) 1395 / x = 15
Multiply both sides by "x":
1395 = 15 * x
Divide both sides by 15:
x = 1395 / 15

Now you have the solutions for each equation.