Consider the equation f=4x−2x+1. This equation has a maximum for x=1.5287 This poin is the critical point obtained solving dfdx=4−21+xln2=0 giving x=ln(4ln(2))ln(2)−1=1.52877. Substituting this value in f(1.52877)=0.344285. This value is lower than 15 so no real solution is expected.