mittels Sage: https://doc.sagemath.org/html/en/tutorial/tour_algebra.html
a, b = var('a, b')
eq = [ a * 5 + b * 3 == 25, a * 7 + b * 1 == 3, a * 9 + b * 4 == 31, ]
sol = solve(eq, [a, b], solution_dict = True) sol
subs = sol[0]
6 * subs[a] + 6 * subs[b]