So, to factorise something you are taking out the highest common factor.Here, 10 is an example of the highest common factor (I’ll refer to this as HCF)
10(2k+5)
Everything inside the brakets is multiplied by what’s directly next to it (if there is a negative sign, then that is included). So, 10 x 2k = 20k and 10 x 5 = 50
10(2k + 5) is the factorised version of 20k + 50