In this question, it is given that the diagonal of the board is 18 inches long and one side of the board is 12 inches long.
Let the other side is of length b inches .
Now we use pythagorean identity, which is
Here, a = 12 and c=18
Substituting these values, we will get
And the formula of perimeter is
Substituting the values of the two legs, we will get