Answer:
You can you the
formula by substituting

Explanation:
g = groups of tennis balls
one can can hold 3 tennis balls
plus we have 1 let over
so you can use an equation like this:
number of tennis balls
So we know how many tennis balls fit into one can and we know the left over amount:

She had 16 tennis balls in total.
Its essentially a y=mx+b equation but replace: x with the number of cans, m with the amount of tennis balls that goes into each can, and b with the left over tennis balls.

And y would be the amount of tennis balls we have in total.