Answer:  
a. write an expression for the perimeter 
perimeter of rectangle = 2(length x width) 
 = 2[(k+5)+(k)] 
=> 2k + 10 + 2k 
 = 4k + 10  
so, a = 4k + 10 
b. find the value of k 
=> 4k + 10 = 50 
 4k + 10 - 10 = 50 - 10 
 4k = 40
 k = 40/4
 = 10
so, b = 10 cm
i suck at explaining but i hope it make sense