book Maths Puzzle Numerical Ability

My question is:
A 26-foot ladder is leaning against a building. The foot of the ladder is 10 feet away from the building. How far above the ground does the ladder touch the wall?

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book Other Question

SCRATCH WORK PLEASE
5x+t=17
7x-k=8
4-2x=g
15+3x=u
a+6x=c
b-7x=n
ax-b=k
hx+j=r
2x=4a+2b
I do not understand how to do the Quadratic Equation r2(squared) + 10r + 9 =0