Help please explain how please please, please

Split the problem like this.
you were given that each square is 1cm.

so for the rectangle on the left it goes as followed
a= lxw
a= 3 x 5
a= 15

and now do the rectangle on the right
a= lxw
a= 4x2
a= 8

now just add the two together
a= 8+15
a=23cm