Answer:
I will not give you an answer, but I will give you how too get the equation started.
Explanation:
set up an equation using: 
 
x= amount invested at 3% 
 
7800 - x = amount invested at 12% 
 
.03(x) + .12(7800 - x) = 504 
 
.03x +936 - .12x = 504 
 
-.09x = -432 
 
x = 4800