I HATE this math

What does independent variable and dependent variable mean in math?

Triangles abc and dfg are given. find the lengths of all other sides of these triangles if: b ∠a≅∠d, ab·dg=ac·df, ac=7 cm, bc=15 cm, fg=20 cm, and df-ab=3 cm.

What is a correct first step in solving the inequality-4(3-5x)> -6x+9