59.4cm Use Pythagorean theorem to find the diagonal (^ means exponent) a^2 + b^2 = c^2 42^2 + 42^2= c^2 1764+1764=c^2 3528=c^2 square root both sides c= 59.39696962 round and you get 59.4 cm
Explanation: to find the answer you need to know the surface area of the box. So you use the formula SA = 2ab+2bc+2ac A=7 B=10 C=5 After inputting the values you get: SA = 2(7)(10) + 2(10)(5) + 2(7)(5) and after simplifying: SA = 140+100+70 SA=310