For 2a, we plug 3 in to get (4*3+3)/(3^2-7)=15/2. Since 2 is a prime number and 15 is not divisible by 2, this is simplest form. For b, using PEMDAS, division goes before addition or subtraction and 3/3^3 would therefore be done and you could not add 4*3 beforehand (or subtract 7).
For 4a, we plug it in to get sqrt(25-(-3)^2)=sqrt(25-9)=sqrt(16)=4
For b, since 13^2=169 and 25-169 is negative, we cannot find the square root due to that nothing squared is negative.
