Question 242932
Good to see you have had a good go at this problem!

Let K = Kathy's age and L=Lisa's age

{{{K = 2L}}}

{{{K+3+L+3 = 42}}}

{{{K+L+6 = 42}}}

Subsitute K/2 in for L:

{{{K+(K/2)+6 =42}}}
{{{K+(K/2) =36}}}
{{{(2K+K)/2 =36}}}
{{{3K/2 = 36}}}
{{{3K = 72}}}
{{{K = 24}}}

Does that help? Make any sense?