The figure appears to be an irregular hexagon, and the perimeter is found by simply adding the length of its sides, which is what the given expression appears to be. Because there are variables, or indeterminates, amongst these, we need to group similar terms together: 15+5x+9z, (that is: (7+8)=15, (2x+3x)=5x, (4z+5z)=9z). The expression 15+5x+9z represents the perimeter and cannot be reduced further, although brackets can be used where there is a common factor: 5(3+x)+9x, or 3(5+3z)+5x.
The figure could also be a triangle with sides: 15, 5x, 9z.
Without further information we cannot find x and z, but if the figure is a triangle, and if the triangle is equilateral, we can equate the sides: 5x=15, so x=3 and 9z=15, so z=15/9=5/3=1 2/3.