Can someone explain how to solve this?

21.5

Step-by-step explanation:

x is the radius of the circle.

If we draw a line from the circle's center to either end of the horizontal line, that line will also be a radius, so it will have length x.  This forms a right triangle with side lengths 10 and 19 and hypotenuse x.

Using Pythagorean Theorem:

c² = a² + b²

x² = 10² + 19²

x² = 461

x ≈ 21.5

13

step-by-step explanation:

169-1=168

168-3=165

165-5=160

160-7=153

153-9=144

144-11=133

133-13=120

120-15=105

105-17=88

88-19=69

69-21=48

48-23=25

25-25=0

since it took 13times to completely subtract it hence its square root is 13