We can solve this graphically
See here : https://www.desmos.com/calculator/zpcbhs2jhw
The first two constraints form a 1 x 1 square.....area = 1
The last equation forms a triangle in this square with a base of (1/2) and a height of 1
Its area = (1/2) (1/2) (1) = 1/4
The probability is ( 1/4 ) / 1 = 1/4