Question 281396
I am pretty sure the only way to solve this at the level I think you are in is using the formula:

If you have a right triangle, then {{{A^2 + B^2 = C^2}}} where C is equal to the hypotenuse (or diagonal)

so with your example, we are given the diagonal and either A or B, it doesn't matter since A and B are commutative.

I will let B = X (or the answer)
which implies
{{{28^2 + X^2 = 35^2}}}

evaluating the squares:
{{{784 + X^2 = 1225}}}

to solve for X, we will want to get it by itself by subtracting 784 on both sides:
{{{X^2 = 441}}}

Finally, square root both sides

{{{X = +21}}}  or {{{X = -21}}}

Since we are talking about measurement, {{{X = 21}}} will be your answer.