Aight cool one tho, love space invaders, thanks for this one.
Doing the right equation is tricky, as my first number picked was not a multiple of the result. I like so much your disclaimer of feedback and wanting to learn more, as this shows a good spirit.
Some feedback and bugs:
I realised the ship collider its a little too big - I have been hit in my left side when a projectile went through, also collected numbers too early by trying to shoot them right below.
All equiations go with "? + ? * ? = r". That multiplication its too tricky, as you may get a random result too low not to be multiplied for something not bigger than 2. You could try to start with a "? + ? = r" equation and follow with the original one (or even go for "? + ? - ? = r" and so on).
Stay hard! :)