Question 7997
Since you don't know how many red cars there are, Red is X.
Since you do know how many blue cars there are, Blue is 2x+3 (twice red X plus 3) 
The total cars is 24.

The equation is:{{{x+2x+3=24}}}
Factor the equation {{{3x+3=24}}}
Subtract 3 from both sides {{{3x=21}}}
{{{x=7}}}

Red x= 7
Blue 2(7)+3= 17
Total = 24

I hope that helps you
CharStar