Write a system of equations using A and B as the variables
The first equation is the sum of the lengths of the plants

The second equation is the relation of growth that plant A grew 5cm more than plant B

Insert the second equation into the first one to solve for B

Solve the equation for B

Use the value of B in the second equation to find the value of A

Plant A is 23 cm long on Tuesday and plant B is 18 cm long on Tuesday.