Steps to solve:
15 + 2a = 35
~Subtract 15 to both sides
2a = 20
~Divide 2 to both sides
a = 10
Best of Luck!
1/2 as an exponent means, square root of the number, so when you have a number with the exponent of 1/2, you should find the square root of the number
In short, Your Answer would be Option D
Hope this helps!