Primary trigonometric ratios
SOH CAH TOA: choose the ratio that uses the two sides you actually have.
A worked example
With opposite 8 and adjacent 15, tan θ = 8/15 = 0.533, so θ = tan⁻¹(0.533) = 28.1°.
The mistake people actually make
A rise-over-run question needs tangent, not sine, because the run is the adjacent side and not the hypotenuse. Use the inverse function when you want the angle rather than the ratio.