x = 58/3 = 19 1/3 = 19.333333
I'm attaching a screenshot showing this rule.
I'm using the letters from the rule shown on the screenshot bc this is the common formula used.
AB x AD = AC x AE
In your problem:
AB = 16+26=42
AD = 16
AC = 18+x
AE = 18
Substitute:
AB x AD = AC x AE
42 x 16 = (18+x)(18)
672 = 18x+18*18
672 = 18x + 324
672-324 = 18x
348 = 18x
x = 348/18 = 58/3 = 19 1/3 = 19.333333
Check:
AB x AD = AC x AE
42x16 = (18+(58/3))(18)
672 = 672