var('a,b,k,l')
eq=[(a-4)*k+b*(-k)==0,    #  eigenValue=4, eigenVector=(1,-1)
    (a-2)*l+b*(l)==0]     #  eigenValue=2, eigenVector=(1,-1)
B=solve(eq,a,b)
B
B[0]
B[0][0]

z = B[0][0]

z.rhs()

sage_eval("2^3 + 5")