board pieces

Let each short piece be  x cm  long.

Then, each longer piece will be  x + 3 cm long.

If you add the lengths of the three short piece to the two longer pieces, the total length will be 150 cm.

--->   (x) + (x) + (x) + )x + 3) + (x + 3)  =  150

Dropping the parentheses:

--->   x + x + x + x + 3 + x + 3  =  150

Adding the like terms:

--->   5x + 6  =  150

Subtracting 6 from both sides:

--->   5x  =  144

Dividing by 5:

--->   x  =  28.8 cm (for each short piece)

--->   x + 3  =  28.8 + 3  =  31.8 cm (for each longer piece)