I assume you're asked to find [tex]7^{4k}\pmod{100}[/tex] for positive integer k. To start, notice that
[tex]7^4 \equiv 7^3\times7 \equiv 343\times7 \equiv 43\times7 \equiv 301 \equiv 1\pmod{100}[/tex]
Then for every k, we have
[tex]7^{4k} \equiv (7^4)^k \equiv 1^k \equiv 1 \pmod{100}[/tex]