4^(x-3)=-51-3(4^x) or (4^x)-3=-51-3(4^x)?
Let's start with 4^(x-3)=-51-3(4^x); 4^x/4^3=-51-3(4^x); 4^x(1/64+3)=-51; 4^x(1+192)=-3264; 4^x=-3264/193. No solution because a positive number to any power is positive and the right side is negative.
The second equation: 4(4^x)=-48; 4^x=-12. No solution for the same reason as before.