Half the line of RE is shown as 12. The line is cut in half by line HS so the top half of the liken is also 12
RH is the hypotenuse of a right angle so use the Pythagorean theorem:
RH = sqrt(20 ^2 + 12^2)
RH = sqrt( 400 + 144)
RH = sqrt(544)
Simplify to
RH = 4sqrt(34)