a. $21$ b. $21$ is a solution of the equation.
Work Step by Step
$a.$ Replace x with 1 and y with 5: $3(2x+ y)=$ $=3(2\cdot 1+5)$ $=3(2+5)$ $=3(7)$ $=21$ b. If the number $21$ is the solution to the equation, then replacing z with $21$ should result in a valid equation (the left side should be equal to the right side of the equality sign) $4z-30=54\quad?$ $4(21)-30=54\quad?$ $84-30=54\quad?$ Yes, the equation is valid, $21$ is a solution to the equation.