You simply have to multiply both sides by 4. In every inequality you can multiply both sides by a positive number, and this won't affect the inequality sign:
[tex] \dfrac{k}{4} \geq 6 \iff 4\cdot \dfrac{k}{4} \geq 6\cdot 4 [/tex]
And obviously the last inequality can be written as
[tex] k \geq 24 [/tex]
