Algebra
posted by G .
X/10+1.7=3.5
X/10+1.71.7=3.51.7
X/10=5.20
X/10 . 10/1= 5.20 . 10
Why is the answer 52?

Because X/10 = 5.2, which you already correctly derived.
The last step is to multiply both sides of the equation by 10.