Mathematical Induction Question - forgetting a simple rule?

I'm working on a mathematical induction problem for a Computer Science class and I'm trying to solve:

a. $$\frac{3(5^{k+1}-1)}{4} + 3(5^{k+1})$$

and I'm getting stuck at:

b. $$\frac{3(5^{k+1}-1+4 \cdot 5^{k+1})}{4}$$

I need to get to:

c. $$\frac{3(5^{k+2}-1)}{4}$$

I'm not having any real problem with the induction part, I'm getting hung up on the math.

I'm pretty far removed from my last real math course and I'm sure I'm just forgetting a simple rule. I have the solution in front of me and it jumps from b to c but I cannot jump along with it.

Any help would be appreciated.

Answer

$\frac{3(5^{k+1}-1+4 \cdot 5^{k+1})}{4}= \frac {3[(1 + 4)5^{k+1} - 1]}{4}=...$

Hint: $1+4 = 5$.