Isn't that obvious what ROJO says by a = angle(sprite.x, sprite.y, mouse.x, mouse.y),
Which is 'a' is just variable (instance variable or global variable or local variable)
or you can write it down directly like:
Set velocity to 100*cos(a), 100*sin(a)
equal to
Set velocity to 100*cos(angle(sprite.x, sprite.y, mouse.x, mouse.y)), 100*sin(angle(sprite.x, sprite.y, mouse.x, mouse.y))