Ellie is building a dollhouse and she has four boards according to the question.
One long board is 7 inches longer than THE TOTAL LENGTH of three of the short boards.
Therefore, if she has boards A, B, C and D then board A equals the addition of boards B, C and D plus an additional 7 inches. We can put this into pictures as shown below;
From the pictures shown, whatever are the lengths of b, c and d, e simply add them up and add 7 to the result to derive the length of a.
B = b
C = c
D = d
A = (b + c + d) + 7