Answer
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.
