Need help for face maths!!!

I know how to do all the triangle calculation but don't know when u use them can anyone tell me what formula to use to find certain things eg u use Pythagoras to find a length

Pythagoras' Theorem and simple trigonometry (SOH CAH TOA) are only useful for right-angled triangles. For non-right-angled triangles, you'll need to use the Sine Rule and the Cosine Rule. These can be used to find both lengths and angles. For finding areas, you have the formulae 1/2 * base * height and 1/2 ab sin(C) (i.e. 1/2 times the product of any two sides times the sine of the angle between those two sides).