Let,
The first triangle with sides 7 and 3.5 be ΔABC
The second triangle with sides 3 and x be ΔCDE
ΔABC ≅ ΔCDE (they are congruent) as:
1) They have a common point C
2) ∠ABC = ∠CDE = 90° (given)
Hence we can say,
AC/CE = BC/CD
= 7/x = 3.5/3
Therefore,
x = (7*3)/3.5
= 21/3.5
= 6
Easyyy