Use a cheap broom for the “scrubbing”! Dollar tree is a good option if you have one but Amazon has options as well.
A 50/50 solution mix of blue Dawn dish soap and white vinegar is the best cleanser ever!
50/50 means for example 1 cup Dawn and 1 cup vinegar. Or just eyeball it and put equal amounts in.
I usually use an empty water bottle and pour the Dawn in with the white vinegar.
Shake it up before using but there’s enough for several uses even in this small bottle.
Pour some into the tub…
Use your clean broom to swish the solution all around the bottom sides and all. Using the broom is much easier way to scrub then kneeling.. I keep this broom specifically just for cleaning the tub out.
It does a very good job cleaning the scum in there and even better if you do weekly. If it’s been awhile then scrub the solution all around and let it sit for awhile before rinsing.
Then simply use the shower head to rinse everything including the broom. I really recommend a removable shower head- it makes life much easier and they are not too expensive on Amazon!
As a side note when I mop the kitchen a broom like this helps those areas that need some scrubbing.