I think it would be more compelling with a metagame offering an incentive to play. Edutainment games in which solving the equation slays the zombie are general

I think it would be more compelling with a metagame offering an incentive to play. Edutainment games in which solving the equation slays the zombie are generally lame, but something along the lines of achievements might work. You could separate the problems into classes -- the sort of thing that could be done with no algebra knowledge (3 + x = 8), simple manipulation (2x + 5)^2 = 6, factoring, quadratic formula, requires clever tricks with logs, requires a numerical approach such as Newton's method, etc. Heck, you could go all the way to Diophantine equations. In parallel, people could select problems to attempt and earn bragging rights. "Solved a transcendent equation in 90 seconds", "Solved eight factoring problems in a row", that sort of thing. Give people a sense of progress and achievement and maybe something to show off. They might be motivated to learn how to tackle harder problems, or jealous of their peers who can. I might play it just for fun at that point. Perhaps monetize it by letting people spend a buck to have a public display space with what they've done?