easy
you need pythagoraen theorme
a²+b²=c²
where a and b are legs and c is hyptonuse
we assume that it is a right triangle
we can easeily see that the bottom is 4 units
left is 3 units up
a=4
b=3
4²+3²=c²
16+9=c²
25=c²
sqrt both sides
5=c
the diagonal side is 5
3+4+5=12
12 units is perimiter
b is answer