Ok, all of these are relativaly simple.
The formula you will be working with for all of these is a^2+b^2=c^2
Im not going to give you the answers only explain how to do it.
#1. 28 and 21 are a and b. Insert them into the formula to get 28^2+21^2=c^2
Solve for 28^2+21^2 and get 1225
now you are at 1225=c^2
Your answer will be the square root of 1225.
#2. In this problem 6 and 7 are a and b.
Put them in the equation above to get 36+49=c^2
Your answer for this will be the square root of 85
#3. In this problem 2 times the square root of 3 is c and the square root of 3 is a.
Put them into the equation above to get the square root of 3 squared (3) + b^2=2times the square root of 3 squared (2 times the square root of 3.
This gets us to the Square root of 3+ b^2=2 times the square root of 3
Minus Square root of 3 from right side to get b^2 equals the square root of three. so b equals the square root of the square root of three
Have A nice day!