Although Homer does not start out with a natural proclivity towards math, that does not stop him from pursuing his interest in rockets. Throughout the story, Homer takes various actions to make up for his initial mathematical shortcomings.
First off, Homer builds a team that includes more proficient math students. Quentin, Roy Lee, O'Dell, and Sherman all get higher marks in math than Homer, and he understands that with them on board, he will be better able to succeed with rockets. Together, with Quentin in particular, the boys study books on rocketry, and Homer starts to better understand the mathematical basics of his pursuit. The book that Miss Riley lends them, Principles of Guided Missile Design, is particularly helpful.
Unfortunately, these books presume that the reader already has a better understanding of math than Homer actually does. Homer commits himself to learning the math that he needs to know to master rocketry. This involves learning calculus, something that his school does not offer. Homer approaches his teachers to ask that calculus be added to the curriculum. Although Miss Riley lobbies hard for the new class, the idea is turned down. Homer then turns to his father's old math books on calculus. Homer even asks his dad to help him study, but is turned down. Yet throughout his pursuits, Homer eventually does learn the math that he needs.