yellow = red – 3 and green = yellow x 2 and red = yellow + 3

or

green= (red-3) x 2

Three fewer yellow than read is the same as saying red -3

twice as many green as yellow is the same as yellow x 2

if green is yellow x 2 you can substitute yellow for the equation to find yellow

you can flip the equation for yellow around by doing inverse operations. To have the equation equal red you need to move the three by doing its inverse. The opposite of subtraction is addition so you’d add 3 to each side, getting rid of the -3 and isolating the red to make the equation red=yellow+3