For 15 just remember the formulas for surface area and volume of a cone. Both formulas depend just on the radius and the height. Since you know the radius and that both surface area and volume are the same, you can set up an equation for the height.

For the rest of the problems, if you want to see the graphs, remember there are free online tools available, like https://www.geogebra.org/3d. There you can input the equations of the cones, planes, etc. and see them intersect each other.

Remember the intersection of two or more of this objects occur when their equations are satisfied at the same time. For example, for problem 24 if you want to see the equation of the parabola obtained by intersecting the double cone $x^2+z^2 = (y+3)^2$ and the plane $y = x$, you can plug in $y = x$ in the equation of the double cone to obtain an equation in two variables ($x$ and $z$).