The first step would be to isolate a variable (solve for a variable)....so u will have something to sub into the other equation
What’s the question ? & need additional information
So the equation looks like this : 15*3x=78
subtract 15 from both sides: -15 -15
then divide by 3 from both sides
to isolate the variable: 3x=63
3/3x=63/3
x=21 (your mystery number is 21)
