Question 314277
You want to solve the equation for A but there is no CAPITAL letter A in the equation.  I assume that you want to solve for little a.

c = sqrt{a^2 + b^2}

Square both sides as step one.

By squaring both sides, you will remove the radical from the right side of the equation.

c^2 = [sqrt{a^2 + b^2}]^2

c^2 = a^2 + b^2

Subtract b^2 from both sides as step two.

c^2 - b^2 = a^2 + b^2 - b^2

c^2 - b^2 = a^2

NOTE: + b^2 and - b^2 cancel each other out.

Now take the square root of both sides.

sqrt{c^2 - b^2} = sqrt{a^2}

sqrt{c^2 - b^2} = a

Done!