Use Pythagorean Theorem of a^2 + b^2 = c^2 where a and b are the legs of the triangle set up by the house height and the ground, and c is the hypotenuse or how long the ladder is. C is going to be our unknown.
Just plugging in you get 25^2 + 35^2 = c^2. Simplify to 625 + 1225 = c^2. Simplify again to 1850 = c^2. The square root both sides to isolate the variable c. C = sqrt(1850) or approximately 43.0116 feet if rounded to 4 decimal places.
The ladder is approximately 43.0116 feet long.