Imagine that you have an imaginary toolbelt on, in the tool belt there are things YOU SHOULD NEVER FORGET, these are your tools for math that you will needs for most math classes.
In this tool belt you will have things like the quadratic formula and pathegorean theory.
The pathegorean theory (a^2 + b^2 = c^2) is a theory which you can use to find any side of a right triangle provided you have 2 out of 3 of the sides.