Answer
$z\geq13$
Work Step by Step
$6z-15\geq4z+11\longrightarrow$ subtract 4z from each side
$6z-15-4z\geq4z+11-4z\longrightarrow$ subtract
$2z-15\geq11\longrightarrow$ add 15 to each side
$2z-15+15\geq11+15\longrightarrow$ add
$2z\geq26\longrightarrow$ divide each side by 2
$2z\div2\geq26\div2\longrightarrow$ divide
$z\geq13$
