problem with equation solving



I'm trying to solve a integration equation, but there is something wrong
with it and I can't understand it.
Does anyone can tell me where the problem is? I am a beginner of maxima.
Many thanks!!

The code is like this:

f(x):=quad_qags(x*k,k,0,1)[1]$
g(x):=romberg(x*k,k,0,1)$
load("mnewton")$
mnewton(f(x)-0.5,x,1);
mnewton(g(x)-0.5,x,1);

Variable of integration not a variable: 1
#0: solve_by_lu(eqs=[quad_qags(1.0*k,k,0,1,epsrel = 1.0E-8,epsabs =
0.0,limit = 200)[1]-0.5],vars=[h[1]],field=complexfield)
#1: mnewton(funclist=quad_qags(k*x,k,0,1,epsrel = 1.0E-8,epsabs = 0.0,limit
= 200)[1]-0.5,varlist=x,guesslist=1)(mnewton.mac line 113)
 -- an error. To debug this try: debugmode(true);
diff: variable must not be a number; found: 1.0
#0:
mnewton(funclist=romberg(k*x,k,0.0,1.0)-0.5,varlist=x,guesslist=1)(mnewton.mac
line 111)
 -- an error. To debug this try: debugmode(true);