C++ error C2297: '+' : illegal, right operand has type 'double (__cdecl *)(double)' [on hold]

I don't really understand why I get this error. this is the code (linear quation)- I get it for the second line here:

solA1.x+=0.01;
solA1.y=m*(solA1.x-x1)+y1;

while 'm' is calculated earlier (is also double):

void linearMovement(double x1,double x2, double y1, double y2)
{

 m= (y2-y1)/(x2-x1);

}

any help on why do I get it and how to fix it?