Question 1026331
Hi there,
Consider the height, the width 
and the diagonal are a right
angled triangle.
Height^2 + width^2 = diagonal^2
42^2 + 18^2 = diagonal^2
1764 + 324 = diagonal^2
2088 = diagonal^2
Diagonal = √2088
Diagonal = 45.69 inches (2 decimal places)
Hope this helps :-)