If you can factorise a quadratic that is the easiest way to solve it. Some quadratics don’t factorise, so then we can use the formula. This video from jayates shows how to do it. If you are studying GCSE Higher you are currently given the formula in the exam so you don’t have to learn it off by heart, but don’t forget to refer to the formula sheet at the front of your exam paper when you need it.
Maths is fun explains this well but don’t worry too much about the “imaginary numbers” at the end. If you are doing Higher you don’t need to know that yet.
Here is a worksheet that you can print off and practice solving quadratics with the formula.