Every stereometry problem is a short sequence of things you already know. We take them one at a time, and you collect the tool each step needs.
A cuboid ABCDA1B1C1D1 has AB=6, BC=8 and AA1=12. Find the angle between the space diagonal AC1 and the base plane ABCD.
AB=6BC=8AA1=12find the angle AC1 makes with the base
The figure is not finished. The box is drawn, but the triangle you need is not. An angle only becomes findable once it sits inside a right triangle — so your first job is to build one.