In the right triangle, there is a relation between its sides
(hypotenuse)^2 = (leg1)^2 + (leg2)^2
The hypotenuse is the side opposite to the right angle
leg1 and leg 2 are the sides of the right angle
In the given figure
The hypotenuse = 18
leg1 = 4
(18)^2 = (4)^2 + (leg2)^2
324 = 16 + (leg2)^2
Subtract 16 from both sides
324 - 16 = 16 - 16 + (leg2)^2
308 = (leg2)^2
Take square root for both sides
17.54992877 = leg2
Round it to the nearest tenth
leg2 = 17.5