All humans struggle, until they've gained sufficient experience to encode and recognize patterns -- thus remembering what to do.
Here's a basic fact from algebra, to keep in mind regarding equations containing unknowns (symbols representing variables and/or parameters).
If you have only one equation, then it can be solved for only one of the symbols in terms of all the others.
In other words, if you desire to solve one equation to find some numerical result (like: "what sales level"), then you'll first need to obtain (and substitute) numerical values for all of the symbols but one.
And, anytime you're given a graph with an equation, look for points where the (x,y) values are evident. Both the graph and the equation define the same relationship between y and x, so graphs are a great resource for experimenting with equations because they provide values to substitute.
With practice, this all becomes automatic. Then, we struggle with the next concepts until they become encoded. Our math toolbox (brain) grows.