So this is the errorcode I get and my code is

> r := (x1, y1, x2, y2) options operator, arrow; (sqrt((x2-x1)^2+(y2-y1)^2))^2

> hreg := (A, C, x) options operator, arrow; A*x+C

> hpconf := (Q, x1, y1, x2, y2) options operator, arrow; Q*(1/4)*ln(2.25*b*K*t/(S*r(x1, y1, x2, y2)))/(3.14*K*b)

> he1 := (x, y, C) options operator, arrow; hreg(A1, C, x)+hpconf(Q1, x, y, xp1, yp1)+hpconf(Q2, x, y, xp2, yp2)+hpconf(Q3, x, y, xp3, yp3)+hpconf(Q4, x, y, xp4, yp4)

> C1 := solve(he1(-75, 0, C) = 20, C);

> plot3d(he1(x, y, C1), x = -75 .. 75, y = -75 .. 75, style = surfacecontour, axes = box);

I have defined all the constants, and squareboxes shouldn't be needed for the plot, right?