27 inches
Step-by-step explanation:
To find the length of the diagonal, we just need to use the cosine relation of the 48° angle.
The adjacent side to the angle is the height of the canvas, and the hypotenuse formed is the diagonal of the canvas. So, we have that:
cos(48) = height / diagonal
0.6691 = 18 / diagonal
diagonal = 18 / 0.6691 = 26.9 inches
Rounding to the nearest inch, the diagonal of the canvas measures 27 inches