Hello!
You use the Pythagorean Theorem to solve this
[tex]a^{2} + b^{2} = c^{2} [/tex]
Put in the values it tells you
[tex] a^{2} + 1^{2} = \frac{5}{4} ^{2} [/tex]
Square the numbers
[tex] a^{2} + 1 = \frac{25}{16} [/tex]
Subtract 1 from both sides
[tex]a^{2} = \frac{9}{16} [/tex]
Take the square root of both sides
[tex]a = \frac{3}{4} [/tex]
The answer 3/4 or 0.75
Hope this helps!