exam Maths Puzzle Numerical Ability

This is from the Chapter 4 - test, and I have no notes or examples.

s=35 + 0.5s

just not sure what I do.
Thanks

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exam Other Question
Factor completely:
8x^2 – 200y^2 Simplify the following completly as possible

(Square root) 18x^5/y^2


Heres my work...
3x^2(square root)2x/y=2x^2/y (square root) 2x


Thanks