you can use Pythagorean theorem or trigonometry
since two sides are 5, then this is an isosceles right triang
the other side must be the hypethenus
so we can use trig functions
any will do
sin(45) = 5/x call third side x the missing one
45 because that's the measure angle of right angled isosceles (besides the right angle itself)
Ok so now you need to know what is sin(45) depends if you remember this special angle
sin(45) = root(2)/2
then we root(2)/2 = 5/x
solve for x
x = 10/root(2) = 5 root(2)
Perhaps Pythagorean theorem is easier to use, it's up to you
but here how you can start name the missing side x
remember the Pythagorean theorem is as follows
a^2+b^2 = c^2 for any right triangle whose sides are a,b,c
in this case a=5 and b=5
so we have 25+25 = x^2
solve for x again, you can continue this one